Try Maplin Electronics on Carr Street. (Across the road from Woolworths and The Co-op). They sell small round magnets, which are fairly powerful, for �1.29.
The stockfinder function, on their website, shows that they're out of stock of the 10mm diameter ones but they've got 2 of the 6mm ones and 1 of the 15mm magnets (which is 20p dearer, at �1.49):
http://www.maplin.co.uk/module.aspx?ModuleNo=2 6348&doy=30m5
An alternative way to get hold of a powerful magnet is simply to take a loudspeaker apart. If, like me, you're a hoarder of old radios and hi-fi speakers, you'll have no problem finding one. Otherwise, charity shops often sell secondhand speakers very cheaply. (There are plenty of charity shops in Ipswich, around Upper Brook Street and Carr Street). The magnets within loudspeakers are often really powerful. (Generally, the bigger the speakers, the more powerful the magnets).
Chris