The end result is splendid. John Woods came down to the BRSC specially to help me check if the boom would clear the bimini and it does perfectly.
A huge thank you to Peter. It turned out (as often seems to be the case) a fiddly and rather time consuming job.
You may wonder why the photo of the U bolt on the bow of my boat. The story is that after sailing hard at the Bridge to Breakers regatta I found the cabin soaking wet. The U bolt had pulled slightly loose and was letting water in. The bolt looked fine to me until Peter pulled it out and it was like a honeycomb - electrolysis. Old age. As often seems to be the case it was not easy getting a bolt that was just right. Even Whitworths don't stock them. However I sourced one that was nearly right and Peter managed to modify it to make it fit tight. Peter also fashioned a splendid bush out of some super duper black plastic (look closely at the photo). Yet another fiddly maintenance task. Thanks again Peter.
PS from Peter
"You forgot to mention the O-rings, although If you had been here when I fitted it I couldn't find ones that fitted perfectly, so I used some slightly larger rubber bushes with a taper on one end and with a bit of lanoline applied they seemed to squeeze nicely into the countersink I drilled on the inside of the nylon pad. I'm pretty sure you'll not have any prob's with it leaking now."
PPS From Peter
"It doesn't matter Pam but I was trying to explain (not very well obviously) that between that black pad and the stem there is a rubber bush (instead of an O-Ring) that has a 45 degree end to it. I have drilled a good big countersink into the back of the black pad where the shafts of the D-bolt or U-bolt (or whatever you like to call it) pokes through it. I then slipped the two bushes onto the u-bolt with a bit of lanolin on it and the 45 degree end facing forward into the countersink so that as I tightened the nuts on the u-bolt it squeezed the rubber hard into the 45 degree hole thus compressing it around the bolt very tightly. I had to take it out and redo the countersink a bit deeper as there was a little too much squeeze the first time. So there you have it fully explained (I hope).
I think that proves my point - these seemingly simple jobs never are. Pam