Yep. Submerge in water when you replace it to check where the leak is coming from. Fill and release valves are the only other candidate but both are easy to fix.
Consider a replacement kit from Lees Precision to keep on the shelf for the future. You'll hardly ever need it but when you do it has all the bits you could possibly want.
Didn't work. Will ptfe it today and if that doesn't work should i try some of this: http://www.screwfix.com/p/fernox-ls-x-leak-sealer-50ml/23614?kpid=23614&cm_mmc=Google-_-Product Listing Ads-_-Sales Tracking-_-sales tracking url&cm_mmc=Google-_-Product Listing Ads-_-Sales Tracking-_-sales tracking url&gclid=Cj0KEQjwhtO7BRCtwuO9gfTH-fQBEiQAdJ8FY4z9qUJXTxNDsBlgXsFx1ni2UZb6RNEv4mrLsxtURDcaAmNC8P8HAQ ??
Consider a replacement kit from Lees Precision to keep on the shelf for the future. You'll hardly ever need it but when you do it has all the bits you could possibly want.