"Conrad" writes:

What's concerning me is that most of the comets and shubunkins and one or
two of the koi seem to spend all their time on the surface under the
waterfall ( they ignore the fountain). Other than gasping at the surface
they appear to be OK, there is no other unusual behaviour and they feed as
usual.
The weather (I am in southern England) makes no difference to their
behaviour.
I have bought some air stones which I am running for 3-4 hours a day. The
fish move away from the waterfall whilst these are switched on but the next
morning they are back to gasping on the surface again.

You received alot of excellent advice already. Another option would be to add
aeration to the filter if possible. I added a spraybar to mine and it solved a
similar problem. It makes for a less noisy pond to aerate the filter if you
prefer quiet sometimes.