I was not going to reply because I am in the US and the OP is in the UK.

I bought a pair (32/40) of Sun polished alloy CR-18 590 rims from eBike Stop a few months back, the had the best price I could find at the time ($24 each). I can not find the polished rims on their site now but they have the unpolished ones. It does appear that Sun only runs off a batch about once a year and when they are gone they become unavailable, or expensive, until the next batch.