Thought I would give an update
I had an engineer look at the engine and he was shocked to notice that the oil pipe from the reservoir to the oil filter had no oil in it. Luckly I had only ran the engine for a very short time. The cause was a kink in the oil pipe, caused by who put the oil reservoir back, put the oil pipe on, then twisted it to refit.
When I bought the boat and engine from a dealer they was meant to have serviced it, but clearly they did not, as it had been just rebuild and cosmetically restored to "as new" condition. The oil filter was less than hand tight, exhaust manifold bolts under tightened and the carbs totally out of balance. Plus various other bolts loose.
The engine now runs like a dream and sounds totally different.
I am now in the process of trying to recover monies paid for servicing etc. The Company are being "its sold with warranty", "we did service it", etc etc. But I now have trading standards involved and they pointed out that a criminal offence may have occurred.