Hi All, and thanks for your comments.
I was confusing myself with the time when I first bought the car and the the key fob had been set up to only open the driver and passenger doors - I had these doors open but had shut the boot with my keys inside a jacket that I had just lobbed in! So I could get in the car but couldn't start it. The AA man did a brilliant job - he spent ages looking at a wiring diagram. then prised open the top rear of the boot and inserted a bladder which he pumped up until he could somehow get hold of the bundle of wires that are on the right side. he cut into the bundle, prised out a skinny little red wire, put a power feed into it, and the boot popped open.
On this occasion I did in fact realise that I might be able to power it up via another car battery, which I did a couple of hours ago.
But thanks again everyone