SlipSlider is right on the Cessna 120/140 association. When you join you get access to their online library of AD's, STC's, and a whole lot of techical stuff. Plus on the forums you have the ear of a bunch of smart C120/140 people. I'm an association member and am restoring a '46 140.
I can't speak about owning a 140 in the UK. However, potential pitfalls include:
doorpost cracks (most have them, no AD against it, there is a cessna service bulletin about them and a readily available STC for a nifty doubler)
Corrosion - typical stuff, but look in the spar carrythroughs.
Cracks in landing gear mount structure
Cracks in landing gear legs where the step bolts on
Tailwheel leaf spring bracket
Empty weight - some of them really pork up with metal wings plus a lot of old avionics. This can be fixed. Or buy a light one.
It's a great little airplane.
-- IFMU