Right, I am back again.
Thanks again for all suggestions and efforts.
Before I came on here I had tried all the obvious places including ASAP and that is why I approached all the wise old heads on this forum.
I have tried all other suggestions that have been put forward that I hadnt thought of myself and still no results. It is definitely not a part I shall be buying of any shelf.
I spoke with a marine engine rebuilder in who was breaking a bmc engine and he was confident he had the pump attachment but when he saw the photos of the 'coupling drive' he said he'd never even seen one.
He was extremely helpful though and couldn't have tried harder to help. So, as of now only one person has said that they have ever seen one.
A gentleman who works at one of the many boatyards I emailed recognised it and was going to rummage his garage over the weekend and see what turns up.
There is another guy from a classic car specialist who was confident it was from a Nuffield tractor engine circa 1960's. I have already been in touch with the classic tractor parts supplier and no luck there either but he wanted a few days to see if he could come up with any suggestions.
It is definitely an engine part, the spigot (small end) inserts 2.5 inches into the engine and engages with something that makes it turn. The wider (end that I cut a piece out of) more than probably drives the taco or something similar and the pump shaft was slid into it and secured with the roll pin and while all the turning was going on the impeller span and pushed water around the engine.
Now, because there are no really heavy forces involved and the holes for the roll pin are still on the shaft, would I be mad in thinking that if I filed off any sharp edges and greased it well, would I get away with just putting it all back together ( including the new bearings that started this whole process) and running it like that. Or I could get someone to weld the small squares I cut out back in place and file very carefully, all back to shape and then run it.Because this thing seems rare and I am struggling.
I have emailed the Thornycroft website but had no reply as of yet.
I think my best bet is to keep looking for an engine that is being scrapped and yes I have tried Boatscrapyards etc.
I am also going visit a specialist engineer who seems confident he can make it but assured me in no uncertain terms it was not going to be cheap.
I have attached some diagrams that show the 'coupling shaft' part number 17 on plate TK.
In the diagram of an engine block it is located 4th description from the bottom on the right hand side, and listed as an auxiliary drive which is very similar to the set up I have.
Anyway thanks all, and I shall keep you posted as to the outcome. But please remember if you see an old BMC engine block in the bramble somewhere, call me as I might need a bit off it :-)