Thank you for the responses - the age of them is certainly a consideration in terms of 'nasties' but then again as you point out, there must be a point where there's v.little left in them (they were never looked after with extra treatments when they were actually used as decking).
I've also sent an enquiry to B&Q to see what they can tell me about the decking they were selling 10 years ago and what their advice would be in using it for raised beds - I'll post any reply I get.