Thanks for the responses guys.
I haven't any idea how old the frame set is, but some of the components are quite old, like a fluted shimano 600 crank and an old campag headset, but the decal looks like a more modern vinyl type and has purple lettering which makes me think its not that old. Anyay it's clearly been used a lot and the chainstays haven't cracked yet. there's no shainstay bridge either. It doen't seem like a very old bike. I think it was probably a custom job.
Another weird thing is that the rear wheel sits slightly off centre but the spacing is 126; so the rear triangle has been bent at some point.
89-94 style decal
ww.reynoldstechnology.biz/wp-content/uploads/2015/10/decal_history.pdf