EDIT 14/11/2023: subsequently found another filter/strainer on the return (correct) that I initially thought was a standard lever shut off valve. It was way worse and jam packed with black debris.
See post below:
@matt-drummer & others
Since I installed ESPAltherma, I can now see flow rates, which seem(ed) stuck around 24L/M - yet I seem to remember soon after installation (September ‘22) seeing just over 30L/M……and have seen other similar units with higher flow rates
I had to ring Octopus in October re: annual servicing, and the earliest they could do was start of Jan ‘24 
So decided to investigate and came across two pretty easy serviceable elements: filter/strainer that when particles accumulate, flow is restricted, and magnetic filter.
Note: Octopus did not power flush/clean the existing system prior to connecting heat pump. (Controversial I now know…)
I used these resources amongst others:
Service Manual for Daikin Altherma 3M
Service Manual altherma_3_m.pdf (7.1 MB)
Mine was filled pretty badly with black gunk (but maybe not as badly as I expected and forgot to photo it prior to cleaning out.
I’ve gone from 24L/M to ~30L/M (yay! But wait til the end…)
Also worth checking two other things:
Is the filter on the correct connection (should be on the return to the unit to protect the outdoor unit - mine below is on the bloody flow/output from the unit!!!
This is the printed supplied documentation that came with the revision of the physical unit we had installed in September ‘22: you can clearly see it matches our unit (quelle surprise), but that it’s fitted wrong way round (as in on the flow rather than return…:
The only minor plus point is the strainer/filter is at least the correct orientation to catch stuff before it heads back into the house….
This is from a PDF version I found that thought was the same, but Daikin have clearly swapped positions of flow/return, and Octopus were (are?!) running off maybe a previous revision/iteration:
Lastly, the Fernox TF1 that Octopus seems to install as standard was also easy to clean out/check, and again not great but not terrible:
