I was going to do the run from the HW and CW tanks in 22mm before
and after the pump to the shower - but if the pump is only 15mm,
is it worth it? Am I better advised to run the whole run (some 10M)
in the smaller size?

Don't think of the resistance in the pipework as being as bad as the weakest
link. Every bit of 15mm pipework adds resistance. Every bit of 22mm pipework
adds a lot less. You will get much less resistance piping in 22mm overall
with such a length even with the 15mm pump.

However, it may be worth calculating the significance of the likely pressure
drop/flow rate, as there are some advantages piping in 15mm. In particular,
you lose less heated water every time you turn off the tap and you get hot
water to the tap quicker.

If it was me, I'd probably use 22mm for a bath (or multiple outlets), but
15mm for a simple shower/basin, as the flow rate is much lower for a shower
and probably can cope with the restriction when pumped.

Christian.