dc42, I was also going to suggest using an old PSU from a puter. There are some very good instructions on the web for converting them to standalone PSU's - Lots of grunt and pretty stable! 
e.g. c/w 3.3V http://www.instructables.com/id/Convert-an-ATX-Power-Supply-Into-a-Regular-DC-Powe/
Lots of other similar pages.
Beware of mains when the unit is open!