I'm not sure that you can use silicone mastic for a permanent fix: it is a non-bonding sealant, so it will peel off after a while. It doesn't really live well with urea (pee).

The ideal would be a tube of Acrylic sealing mastic, that dries semi-flexible. Even better would be a cyanate-based waterproof toilet sealing mastic (light grey, semi-setting) but this is a bit more expensive and poisonous if you don't handle it properly (good ventilation, plastic/rubber gloves). I would still recommend it as the best repair, though. B & Q should have it.