Firstly, it is good to hear of someone with happy practical experience of these things. Perhaps I have always failed to invest enough money in the ones I've tried.

Google 'Black and Decker 450 Amp', as you are searching in the UK you will get UK results. Including the word Amp avoids all the 450 Watt hedge trimmers, etc.
Seems you will pay 60 to 70 UK Pounds for one.
I did pick up one US Ebay advert, for $85, so the $35 purchase sounds good value for money.
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0106974263
The heart of these things has to be the battery, which in this case is 19Ah, if anyone starts making comparisons with other makes. The other 'gotcha' I've found is the ones that cannot themselves be left on mains charge for an indefinite length of time. I know that when in the car they won't be charged, but when back in the Garage or Home, having to recharge for only 2 hours, or 4 hours, or whatever, is a right pain. It seems I rarely remember, so the battery is always damaged through overcharging, or the charger melts!
I can see there is always a case for a portable power supply, but for cars in occasional use, as in this instance, I think the technically better answer would be the sort of battery charger or maintainer that can be left across the battery all the time. Trouble is, you need at least two, and some of them seem to cost as much as the B&D power pack.
Good Luck in your search.