I am looking for an answer to the same question. I have a horrible feeling that we need to change the entire flame failure device: eg http://cgi.ebay.co.uk/RANGEMASTER-55-FLAME-FAILURE-DEVICE_W0QQitemZ120126172408QQihZ002QQcategoryZ43566QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I asked a question on their site, maybe they will confirm that.
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Try fixfault.com to fix gas cookers.