| Suprised Maplin don't do a 5A 12 volt powersupply, could try using the 12V part off a Computer PSU.
Also check Ebay, things like Toshiba robots used quite high current 12V supplies to control their robots, a lot of our equipment at work has 5A plus DC power supplies, there should be some out there, either that or get the bits to build one, full wave rectified and regulated | 
skidlids
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