It may be that either your combi is one of those that regulates its
output flow rate - which in in some ways is good since it limits the
temperature swing, or, it has been fitted with a pressure reduction
valve on its inlet.

I think it has both from the manual

I remembered I scanned them for Ed's collection a while ago, and at
best, it looks like it might do 12.9L/min at a rise of 30 degrees or
7L/min at 57 degrees rise

A mixer that responds to a rise in temp by increasing the cold flow rate
works better in these cases.
Aha - I think that's what I was babbling on about in the previous
sentence - any ideas how you can tell one from the other ?
Some explicitly state they are suitable for combis and multipoint heaters.

Thanks for the replies John, appreciated :-)

I think we might be going for the 89618 :-} (if the flow really is
crap, we can just use the handset over the winter months, or just sit
in the bath !)