After false alarm it was a £2000+ ECU which had failed, turned out to be coil. That is now replaced and she is running well.
45 Knots (2 way average) in the Solent this morning in slight swell and show chop, revs 5150-5350. This is slightly faster than I would have expected with half a tank.
She makes a new relatively high frequency purr over and above the throaty bass noise I was used to, at around 1800rpm = 7kn going up river. Sort might be exhaust noise. No idea why???