Thanks Konrad / Lemur,
Noted on your points... I had already checked similar bikes being sold locally (South London area), and as you said there isn't much more to go off of after that other than the bikes condition...
Luckily the owner currently has the entire bikes worth in spare/ new parts including a clutch and forks which he is also throwing in.
Appreciate your responses! Hopefully I'll make a deal on it soon.