Evening Phil. Get several quotes. They will need to do a heat loss and heat demand calculation. 
Your dimensions and oil usage were similar to my bungalow. So will be interesting to compare the outcome. 
I would ask why they chose the specific make of ASHP? Support. Spares. Service. 
Pipes and flow are important. If you have any small bird pipes, how will they deal with this (or just replace? Which was a no no in my wife’s view)

@julianc

thank you for your response, helpful. What do you mean by small bird pipes. All of my ground floor pipe work is embedded into the concrete floor and pops up at the radiator points from the floor. All copper pipes. My wife would be very fussy if pipes were going to end up a mess and overly visible. They are mostly all hidden now and I will expect a retrofit to ensure that’s maintained. I see and hear of so many poor installations and as this is a comparatively new industry for the U.K. am finding great value in this site and others expertise.

@phil3822 we used JL Phillips from near Newark.  The installation was flawless.  We had a complete system as we had no central heating.  Service I don't know because nothing has gone wrong.  

I'd ask suppliers how long they have been fitting ASHPs and how many they've done before.  If they are experienced, they should be able to make a good guess at the size of ASHP and the cost from the EPC and having a look round (i.e. without doing a detailed heat loss survey).  Other than that, just try and figure out whether they know what they're talking about - they should know a lot more that you!  In terms of the final design, they don't have a lot of leeway as (if you're claiming RHI) the installer has to follow MCS guidelines, which will specify the ASHP size  and individual radiator requirements for each room.  

If you have large rooms you might need multiple radiators depending on the layout.  Our lounge needed three. Although you can replace K1s with K2s with K3s ...  

@phil3822, welcome to the forums. From our experience, the two main areas (which I've repeated many times in responses to homeowners) that you should go through at the proposal/quotation stage are the heat loss calculations and heat emitter guide. These are two very telling documents.

The first essentially ensures you get the correct size heat pump for your house, and the second will tell you whether the heat emitters (UFH and rads) will have enough oomph to emit the heat that is required to get rooms to temperature. This was a massive lesson for us. If you haven't seen the video already, this is what we cover in this video:

We also covered the issues with pipework in the same video, which was a contributor to our poor flow and underperforming heating in several rooms.  

@julianc, is also spot on. You need to find a manufacturer and installer that will offer support after the pump has gone in. For us, Global Energy Systems have been very supportive, but have seen very good comments about Grant UK's after sales support. This is very, very, very important, because these heat pumps can be fiddly and need tweaking, so you need a supportive partner to help and guide, especially for the first two winters.

I would suggest posting quotations and proposals on the site (removing any financial figures and contact details if you wish) so that we can provide insights and feedback from our personal experiences.

Why did you say that a south facing heat pump is not good?

Thanks guys, I have read on an American site and also build hub that south facing air source pumps rubbers perish more quickly. I have also read something about the heat pump internal thermostat but that post was unreliable and about 10 years old. Not sure if therefore is duff info.

my living room is 26ft x 14ft and currently has 2, k2 radiators 180cm x 60cm.

kitchen/dinner is 27ft x 12ft and has a 100cm x 60cm k2 and no space for anything bigger in height or length, hope a k3 does the job but doubt it.

these are the challenges I will pose to the installers plus your own questions kindly added.

Timing for the job is going to be key also. RHI vs potential £4K clean heat home grant and having a new born means wife is on maternity leave so lower earnings and more outgoings so all in the balance. 

Mars, I have watched all of you and your lovely wife’s videos and that led me here. I will use this all as a reference.

Phil, by small bore pipes to the radiators, in my case I mean 10mm. We have large “mains” pipes running in the loft that are 35mm or 28mm with 10mm drops to each radiator. Lots of installers I spoke to said the 10mm would have to be replaced 😳. NO!

I think “small bore” is 8,10,12 mm. 

@phil3822 try this to get an idea of radiators. 

http://starsapp.co.uk/basic-heat-loss-calculator/

The other thing some people recommend is (if you can) to turn your existing heating temp down to 50 degrees and see if your house is still warm. I do think you will struggle with your kitchen though. An ASHP rad @ 50 degrees is going to emit less than half the heat of on oil CH one at 70 deg. A K3 will give you about 1.5 x a K2 so it's not going to make up the difference. We had to lose a kitchen cupboard to fit in our kitchen radiator.    

I wouldn't worry about being south facing.  On a winter's day a bit of sun will help the ASHP. The weather in the UK isn't as extreme as the US.  Another question to ask potential installers.

Thanks again guys. What is a hydro box? In some cases it seems to replace the water tank completely and in others it seems to be worth with the tank. I assume it acts like a boiler but not sure. 

Hi Phil3822,

Welcome to the forum.

The only thing that I would add, that does not appear to have been mentioned by my forum colleagues, is how well insulated is your property.

Improving insulation and draftproofing should be tried before considering upgrading radiators. It would also help improve overall efficiency and reduce energy costs.

@phil3822, a hydrobox is the indoor part of a split system, containing the heat exchanger and a pump.  In a monoblock system (which is what most people seem to have) it's all in the outside unit. That's where my knowledge runs out I'm afraid.  

https://www.griffithsaircon.co.uk/services/renewables/ashp

Hi Phil3822,

If you follow the link below it will show how a hydro box fits into the system.