For those that asked, this is the thing i was using on Sunday with the inverter.
http://cgi.ebay.co.uk/Car-Jump-start-Booster-900A-Portable-power-12v-inverter-/260613492441?cmd=ViewItem&pt=UK_Recovery_Tools&hash=item3cadc64ed9
Not cheap, but works well.
If you look round you may find them cheaper, maplins do a couple of different versions.
Plod11