Thanks for the advice.

I now definitely have signs of a dead fish, oil on the water surface (fixed
by switching in the surface sucker), bits (yuch), some smell (for once the
cleaning siphon is loaded from the tap, I am not sucking on it!). Main
filter not touched, added the powerhead and another small canister filter
loaded with carbon - which I am hoping will remove some of the organics and
smell (I am working on the basis that if it is more comfortable for us, it
will also be for the fish). To date ammonia and nitrites OK (as far as I
can tell from the colour chart, close to zero on both). Fish are hungry,
arerated and 30% water changes ongoing...

I will let you know what happens.

Cheers,

Gary.