Thanks so much for all your advice. It seems clear to me now that an engine survey is absolutely necessary. The previous surveyor didn't get a chance to run it, and sounded pretty perturbed by cooling pipes that were not only cracked but mouldy too. That was two years ago...
After reading your posts I had another look at the hull survey, done two years ago, and the dreaded question of osmosis figured heavily:
'A Tramex SKipper Plus moisture meter set to scale 2 for GRP was used to take moisture content readings. The meter gives readings of 1 to 30 that are proportional for moisture present. Readings of 19 and above are deemed high. It was reported that the vessel had only been out of the water since the previous day, so the hull would have had no chance to dry out. As the boat was 39 yrs old, she would have had an Orthophthalic gelcoat, which takes longer to dry out then the modern Isophthalic gelcoat. Readings were taken at random, and all were over 25, and most were off the scale. Blisters were seen all over the hull, some of these had cracked, especially all around the propeller. Realistically this type of boat and age of boat would be expected to have high moisture readings and blisters, due to the length of time she will have spent in the water, her age and construction. This hull has probably had high moisture readings for a long time. No voids were found when the hull was hammer-tested. A high moisture reading and blisters could imply that the hull had osmosis.'
So all this was done two years ago, and since the only repairs made were antifouling after the survey, it seems rather worrying. Is it the case that osmosis treatment is sometimes so costly that it is simply not worth doing, considering the value of the boat? So the surveyor seems to imply. Could this boat's value deteriorate rapidly due to its potential osmosis problem? How costly can it be? Sorry for all these questions, just need to get my head around this problem before I speak to the seller again...
Thank you again,
Yvo