Do you know what the alarm means.

From what you 'have been told' it sounds like a 'low flow' alarm.  If it wasnt there initially then something has changed to increase the system resistance and thus reduce the flow.  The most likely candidate is a blocked filter (Im basically agreeing with the advice you have received). 

Both gas and heat pump central heating systems have filters to take out the crud (technical term!) that accumulates from the inside of the radiators.  They need to be cleaned out every so often and if they aren't you can get a low flow alarm (or the consequences of low flow) with either.  The recommendation is often to clean them out annually, but in practice they may last years (depending on your emitters) before they become a problem.   That's just how it is with central heating systems.

They are typically roughly cylindrical about 10cm diameter and 20cm high.  Google "Magnaclean" which is one example, or "Spirotech" which is another.  It will be most likely be fitted close to the main unit in the main pipework which is the 'return' ie is bringing water from your emitters to the heat pump, or less likely it maybe just inside the unit (check the manual).  If there isnt one, then the likely consequence is a blocked heat exchanger in the heat pump, which is not good news.  A gas boiler suffers from exactly the same problem.

The next most likely problem is a water pump fault, but if you haven't cleaned the filter then everything points to the filter.

Since these filters are used in most central heating systems any plumber worthy of the title should be able both to find and fix it (or if you can find it then its a simple DiY job).  Of course not all plumbers are worthy of the title.  Feel free to post some pictures in case I or someone else can spot it.

On the matter of running cost do you know what COP you are getting and have you explored alternative electricity tariffs.  There is no reason, with a half decent heat pump set up and an appropriate tariff, why you should be paying anything significantly more than you would pay for a gas fired system, and you might well be able to pay less.

Further to the above I found the manual here

Page 37 describes the alarm and possible reasons, of which a blocked filter is one.

Page 29 describes the particle filter which is, it seems, integral to the pump (but may nevertheless be fitted externally depending on your model - see page 29).  It also describes how to clear it which does not require draining the system (but does require a pair of 'supplied' circlip pliers, probably now discarded but obtainable from eg Halfords)

You may in addition have a third party Magnaclean/Spirotech filter a blockage in which would have the same consequences and cause the same alarm so far as I can tell.  So its worth looking for this as well.

Hopefully that helps.

This really should be an annual maintenance action, but I guess many people (including me) don't actually bother with annual maintenance of heating systems.

PS, for transparency I'm neither a plumber nor a heating engineer.  I have a physics degree and spent 30 years in engineering, so have a bit of a 'nose' for this sort of problem, but essentially zero specialist knowledge.  (This may or may not be a disadvantage, because I know essentially nothing I am forced to work almost everything out from first principles, which generally turn out to be extremely simple and common to multiple systems.  However please note that this means I can also get caught out!)

Many thanks for taking the time to reply. There is a Magnaclean on the return from the rads/UFH, which I have cleaned myself, is this just the white plastic filter and actual magnets that you clean and there is no other cleaning necessary? I have tried to upload a pic, but for some reason won't let me.

There should be a particle filter on both the cold feed and the hot flow and return, as mine is the C line with the hot water tank inside the unit, both are external to the pump, however, I think my installer, who was pretty useless maybe forgot to put them in.

I'm also a mechanical engineer by apprenticeship and degree, which is why I probably know more than the heating engineers that visited. Unfortunately, it's a matter of confidence of not screwing it up completely at this time, as I've managed to get the pump into limp mode by lowering the water temp to 35 and also bring the heat curves down, this way the GT8 temp remains below the cut out temp and I get a little heat into the house. However, the anti-legionnaires function, ie heating the water to 65 will not work.

Thanks for the manual too. I will check if this is the same as the one I have. 

Posted by: @abbote

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There is a Magnaclean on the return from the rads/UFH, which I have cleaned myself, is this just the white plastic filter and actual magnets that you clean and there is no other cleaning necessary? I have tried to upload a pic, but for some reason won't let me.

I can see the magnaclean.  I cant remember whats inside TBH but that sounds about right.

Posted by: @abbote

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There should be a particle filter on both the cold feed and the hot flow and return, as mine is the C line with the hot water tank inside the unit, both are external to the pump, however, I think my installer, who was pretty useless maybe forgot to put them in.

Oh dear - its just possible that the heat exchanger could be blocked then if these aren't present and you have a flow problem.  Is there any way you could flush it through?  Of course there are other possible explanations for a flow problem but filter blockage this is definitely the most likely.

Posted by: @abbote

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I'm also a mechanical engineer by apprenticeship and degree, which is why I probably know more than the heating engineers that visited. Unfortunately, it's a matter of confidence of not screwing it up completely at this time, as I've managed to get the pump into limp mode by lowering the water temp to 35 and also bring the heat curves down, this way the GT8 temp remains below the cut out temp and I get a little heat into the house. However, the anti-legionnaires function, ie heating the water to 65 will not work.

I think you have put your finger on why this might be better as a DiY job!

As I thought I've been told that the most likely cause if the internal flexible pipes below the electrical cassette and flow pump. It's great that the price of these pipes is only £146 inc' VAT but so far the best cost I have for their replacement is £1,000 ex VAT, and that's only for 0.5 days labour. I'm still leaning towards ripping it out. Is there a market for second hand spares or repairs IVT GSHPs?

Posted by: @abbote

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I'm still leaning towards ripping it out. Is there a market for second hand spares or repairs IVT GSHPs?

That's a pity, the most expensive part of a GSHP is, SOFAIK, the underground bit by some margin, which you already have!

Unfortunately we live in a throwaway society where nobody wants to (or knows how to) repair anything, and if they do wants to rip us off big time.  Its true across the whole spectrum of 'consumer' goods.