You need current to start your car not just the voltage. It can be showing 12v+, in 'no load' conditions but as soon as you put it under load that voltage could fall instantly to almost zero if the battery isn't in good condition. it would be a good idea to get a local garage to test the battery under load for you. The price of a new battery isn't frightening.
Welcome to the forum. Seems like you viewed an absolute dog MrLee, could be why it was on a SORN. I am sure you find a good car, although I don't think black is not a common colour. Good luck.
That isn't a daft question Buddy, a rather good one. I use an 8ft wide (you did write 1930s) garage for a Zed - with a careful nearside bias, the wing mirrors fold in and a good intake of breath I can get in/out or the car.
There are plenty with lower mileage than that some people just like to clean and look at their Zed
Sadly; i am one of those, spursmaddave. (2006GT, 15K mles)