Hi, I also have a UK build F21 and my gooseneck looks identical to yours. I’m afraid the picture below is the best I can find at the moment. I also found the following sketch in the paperwork though which could suggest it was remade at some point.
Interestingly the end of my boom doesn’t look the same as the others on this thread:
I measured the foot of my (Lucas) main as 3200. I’m sure my measurement error is less than 80mm but perhaps it has shrunk over time.
My mast has a rubber cylinder (slightly tapered) bonded to the mast which is a snug fit in the hole, then has a rubber ‘hat’ that sits over the top held in place with a large jubilee clip. I don’t have any pictures I’m afraid, but as long as it’s not loose in the hole and water can’t get in, I wouldn’t have thought the details of the joint would matter too much.
Best of luck sourcing the parts you need.