More of a thank you this! I had the Geberit 'blistered washer' problem, and solved it, at zero cost, despite very poor accessibility, thanks to a combination of this blog and diynot.com. Isn't the internet wonderful!!!
Many thanks to the contributors on this topic, and all others of course. I'll try and share my own knowledge, such as it is, from now on.
John Storey, February 2012