I used one of these (plastic) when replacing an old rusty one... seems to do the trick!
http://www.diy.com/nav/fix/plumbing...lose-Coupled-Cisterns-11477959?skuId=11988666[/url]
http://www.diy.com/nav/fix/plumbing...lose-Coupled-Cisterns-11477959?skuId=11988666[/url]