Personally i'd offer a cheeky £15k if it's been laid up for that long, there's no way i'd pay top money for it, you can get decent E60 M5's for £20K, why would you pay more than that for one that could have issues and has no test/tax etc..
Plus depends on circumstanes, why hasn't it been used? Enthusiast owned and kept in a heated garage and not used, or some dodgy character in Jail? All matters.
Haven't seen any decent e60 m5's for 20k just cat d and shitters with 100k on the clock where have you seen em? That's a genuine question not me being smarmy cunt!