sealant for water system??
 

I have removed and refitted a plastic housing from an engine block, unfortunately there seems to be a leak now, there is a rubber O ring, which I plan to change, but was going to use a sealant also to make sure, is there a particular one to get, I'm assuming halfords would be the best place to go for this?

It is on a 205 GTI lump btw :)