It's always tricky when you can't see it working. Bikes like that normally have one fault, which could be wiring, controller, battery or motor, so the probable worst case is £200 for a new battery, which means £420 to get the bike runing by the time you bought the pedal too.
If it has the wiring fault that blows the display and it needs a new battery because it's not been charged in years, you'll end up with a rather expensive used bike by the time you have everything sorted.
Walsham is not going to be easy to get to for most of us, so you need to add in transport costs. It's a shame because if it were near me, I have several of those type batteries, so I could have tested it for you.
In short words, I think it's too risky unless you already have everything to fix it languishing in a shed. They weren't bad bikes in their day - not particularly powerful, but nice and comfortable and very smooth motor. Easy to derestrict too.