Maggots............ you've entered the twilight world of dimming. Even the trade outlets don't always know which one to use. There are so many varieties of lamps around.
12volt
Mains voltage
LED
Halogens etc etc
It's not always just getting the right dimmer......... it also depends on the load. Sometimes, 400w and 500w dimmers just won't work if the load is too small for them to control it properly.
There is a type called an "intelligent dimmer"
http://www.tlc-direct.co.uk/Main_Index/Wiring_Accessories_Menu_Index/Wiring_Accessories~Dimmers_All/Wiring_Accessories~Dimmers_LV/index.html
Screwfix and builders merchants won't have a clue. Try a proper electrical trade outlet such as CEF or TLC. They should know........... possibly.
Check back here later today or this evening. Guru BRIGHT SPARK might see this. He'll know.