Use watch recon, theres always loads of Grand Seiko watches for sale on there, and usually at the great used US prices.
Also, even if you buy from abroad (USA, JAPAN) even after you have paid customs charges the price is still lower than the Uk retail prices.
Best person I have dealt with is a shop called TOKEIKAN on rakuten, he's a master shop dealer for GS in Japan, has great prices, but only speaks very simple broken english.
http://www.takekawa-t.com
http://global.rakuten.com/en/store/tokeikan/