Given that I've read of people having no luck (highly variable) with TWO adapters in the chain, you might be better to just get a FW to USB adapter like this one:
http://www.amazon.ca/IEEE-1394-FireWire-Fe...+to+usb+adapterUSB 3.0 is backwards compatible to FW, so all you'd need is the one cable. It won't be blazingly fast, but then you're limited by the FW 400 speed anyway. Good enough for backups, certainly.
Of course, that said, I'd encourage you to get one of the nice little 2.5" USB 3.0 backup drives available now - much faster, only need the one USB port for connection AND power and very portable, being just slightly bigger than a deck of cards. For example:
http://www.amazon.co.uk/Toshiba-HDTB105EK3...rnal+hard+driveBut if you decide to go the adapter route, here you go:
http://www.amazon.co.uk/Apollo23-IEEE-Fire...+to+usb+adapterCan't beat that price!!
(Unless you find a few other things you want, you'll pay a heck of a lot more in shipping than you're paying for the adapter! Do you happen to have the equivalent of Canada Computers in the UK - somewhere that stocks all manner of computer bits and bobs? Might want to check there, if you do)