I have never heard of anyone successfully repairing one on a long term basis.
As a temporary repair pending replacement, I have used standard bath silicone sealant on the inside but it has to be completely dry and warmed up with a hair dryer to get embedded water out.
If the crack is below (or near) the waterline don't repair - take heed of Softus's dire warning. If it's the lid then Araldite is the boy - if done properly it'll last for years.
Thanks for all the advice. It appears to be just a minor crack (very slow drip round the back, which doesn't seem to come from any of the inlet/outlet seals). I'm hoping a bit of Araldite followed by some sealant should see it through to the next bathroom refit.
If you must repair it, than use a two part putty resin like Milliput. It works in wet conditions and as it is a putty rather than a gloopy resin like Araldite, you can mould and press it into postion until it dries - usually 1 hour.
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