Hi folks, many thanks for all the excellent advise! All taken on board. Mad Hat Man was kind enough to come over and spend a good hour or two having a go at getting the cooling fan off. It won't budge for the moment, I have given it several soakings in WD40 through-out today and will try and purchase a flat long bar tomorrow to brace the coupling.
Decided that drilling this out in situ, at least for me, would be inviting further problems. Going to have a chat with some local professionals and see if I can get one of them to put their hands up for getting rid of the bolt. If the manifold does have to come off, which looks likely, then I will get a new one instead.
On a more positive note, I did manage to spend the day doing many other servicing tasks, removing all the breather hoses, cleaning thoroughly and refitting. Incredible how filthy some of them were, and have obviously never been cleaned in the 13 years since they were first installed.
