@wh0am3ye - do you have so called primary and secondary circuits (ie some form of separation of what goes through the heat pump and what goes through the rads)? If so, I guess the magnetic filter is on the rad circuit, but there is usually also one on the primary circuit. If there is, have you checked that? Mine is in something that looks like a ball valve, but the ball is actually a filter that unscrews from the bottom closing the valve (and reopening after checking/cleaning!).  

@cathoderay As far as I can tell, there is just the one filter, on the heat pump side of the cylinder (bottom left in the photo), I attempted to clean it out earlier on, but wasn't able to remove the mesh inside. Perhaps I need to have another go at that as watching the only video I can find online with my issue, the flow rate was being limited by a blocked filter.

@wh0am3ye - can't make out all the details from that photo - is it separated (primary and secondary circuit) or just one big open circuit?  

Either way, you need to get the mesh filter out of the magnetic filter, blocked mesh filter and low flow rates have a natural affinity!  

@cathoderay Honestly, I'm not sure. The flow and return come from the heat pump, in to the cylinder on the left side and then the flow and return out to the radiators comes out the cylinder on the right side, where the pump is. I don't know if there is a secondary circuit or not. 

Sounds like the plan for this evening is walk the dog, have dinner and dismantle the filter again! I'll update when I've cleaned it and see what happens!

@wh0am3ye - it's just possible that is a huge buffer, the mother of all buffers, not sure. What about DHW, is that entirely separate, with a diverter valve? We can explore more tomorrow.

Meanwhile getting that filter out makes sense. Mine does sometimes need a firm pull to get the mesh but out, it gets a bit stuck at the bottom. If you can find a parts diagram (or cross sectional drawing) for yours, you can reassure yourself that you cam make sure you pull in a way that won't do any damage.  

@wh0am3ye Check out by the ASHP on the pipes next to the unit and see if there is a Y strainer this is usually the culprit to look for 1st

@wh0am3ye, as @ashp-bobba has pointed out, most flow errors on heat pump systems are caused by a blocked Y-strainer. What's remarkable about them is that they don't require a lot of dirt/corrosive bits to cause the issue. This a good starting point.

@editor we now fit 800 micron Y strainers rather than the very standard 100Mic, Samsung and a couple of other makes are very sensitive to low flow and we found when servicing as we don't fit Samsung units that the 100Mic can sometimes halve the flow in a clean system, the 800 reduces it by about 10%.

We also sometimes go as far as fitting 35mm Y strainers on a 28mm pipe with reducers just to give a system less restriction but the same protection, just another example of a littler further for it to be a little better, also we actually go back to our installs and service them year after year so helps us not have annoying low flow issues.

Thank you for all your responses! 
I gave the magnetic filter another clean and got the mesh out this time. Flow rate back up to 24.5l/min! Success! 
I will take a look to see if we have any Y-strainers tomorrow and give them a clean as well if so.

@wh0am3ye super, glad your back up and running.

@wh0am3ye - good news about the flow rate being better after cleaning the mesh filter.

Posted by: @wh0am3ye

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I will take a look to see if we have any Y-strainers tomorrow and give them a clean as well if so.

Bear in mind the filter if fitted may look like a large ball valve with a cap screwed into the base (as mine does) rather than a Y strainer. 

I also think it is still worthwhile identifying the basic design/layout of your system, if only for future reference. I am now more inclined to think that what I thought might be the mother of all buffer tanks may in fact a DHW cylinder, but it is by no means clear. For example, the pump visible in the photo appears to gave been labelled CH1 - which suggests central heating, not DHW.