Thanks for the replies folks!
As there was a common theme of bad connections re- the excitation caps I had another jiggle of the switches (always suspect in a humid environment) but that didn't cure it.
Decided to look a bit deeper and found that I'd managed to screw a terminal down onto insulation on one of the capacitor connections (Doh!) and that the copper end could have been intermittently making contact. It was where two caps were connected (series) and the bad connection went to the motor (via the switches).
The good new is I'm back up in the 240w ball-park, though now at about 28Hz - I'm not too worried about Hz as long as I'm getting the watts (and in fact 28Hz gives me more wiggle room on the transformers) so I'm happy again
though I'm still wondring if I should be looking for another dodgy connection.
I did try wiggling the nozzle to see if the alignment was causing trouble, but it seemed OK.
I don't think I'm going to get more power without more rain though - the penstock's gurgling again so I've obviously got air in it again
. I haven't fitted the smaller nozzle yet as I decided to make some repairs to the washed-out ford and If I reduce the water-flow through the penstock, my temporary chute may overflow and wash away the new concrete.
...It's a cheap simple and reliable generator but not a particularly efficient arrangement and works best when there is plenty of water and you can afford to run the core saturated. For marginal water supplies there are better forms of generator.
I would like to try a PMG - possibly a small, high-rpm axial-flux - at some point but it's a question of finding the time, and it was a choice of either this genny for this winter or nothing until next winter - and 240w 24/7 is cracking-good compared to what I'm getting from the PV right now (had about a 3 weeks of high-pressure grey overcast here recently).
cheers for the help folks.