Basically, today I went and took a look at the P bracket cutless bearing on Forethought looking for wear as it was making nasty noises in astern and also a periodic vibration when in forward gear.

It was not the cutless bearing. It turns out that after the careful realignment of the engine with the prop shaft following the shaft dropping out of the gearbox coupling, I had it pretty straight, within the limit of the semi-flexible coupler. What had happened was that the prop shaft had ended up about 1mm too low where it passed through the rather flexible Volvo shaft seal, as the engine was lower than before when I started the procedure.

It was then occasionally knocking on the wall of the sterntube.

Astern: The nasty noise astern was the cutless bearing groaning with the off axis shaft.
Ahead: The gearbox has a 2.01 to 1 or similar ratio which means that the shaft is not in the same position every time the engine fires to minimise wear. As a result about once every 2 seconds the prop shaft would hit the side of the stern tube as the engine bounces.

I have raised the engine by 2mm to try and see if this cures it. First tests look promising.

In the end when I pulled the engine out to clean up the paintwork, I found the real reason which was that the semirigid coupler had broken.
Part of a piece of metalwork had fallen out.
It was cut through by a slipping propellor shaft rotating in the coupler clamp.
It turned out to be a strap holding two bolts together that took the engine torque. One of the bolts had no thread where the whole lot had vibrated around.

Pete from Marine Power said that once the prop shaft starts slipping its usually impossible to stop it as the shaft burrs so the clamps dont grip properly when you try to tighten them up, the burrs soon give way and it slips again.

Instead of a new coupler I had a new metal strap made by French's of Netley for £15.

I got another 0.5 knots out of the boat under engine and much less vibration.