Thanks for the reply.
I have read that it could also be the guides that have gone, but have seen other forum users have had the old guides bored out and new ones fitted, so this doesn't seem too difficult. You're right though, you need to find someone with the correct equipment to do this.
I did spot a second hand 650i N62B48 on eBay UK, which I am interested in. It's done 68k Miles and I would imagine will be a direct replacement. From a newer '07 650i, but generally the engine mechanics should be the same you would have thought. I will check with the seller this doesn't have any issues with smoking, otherwise no point.
Does anyone know of a company in the UK who can help with the oil seal replacement, and if possible also the guide replacement on both heads?