Judging from information on the nkilgariff site, this has all the hallmarks of a 1983 Mistral... fork lamp boss and seatstay rack brazings included.. the only year when these were
I reckon those wheels are 700c...17" or 17.5" stays.. can't remember but the information is in the 1983 catalogue on the nkilgariff site. FWIW a 1985 Holsworth Special of my acquaintance has about 40mm of clearance between the chainstays at tyre position. I can't imagine this Mistral is gonna be much different.
With respect, Roadking, in 1967 a (super) Mistral like yours was the top of the range Holdsworth factory frameset, with all the extra attention that that implies, but by 1983 it was way down the range. Still 531db throughout... I'm not saying it's not a good frame, but maybe not 'very, very good', like yours! It's a bit like Campagnolo...you know, Gransport was once top of the range too....This may influence opinion as to what this bike is worth. I agree £450 is way too much. Personally I'd be reluctant to go above £200, but I'm not someone who needs a 25" frame, and also I'm a stingy bastard.