I'm guessing you'll be needing a sealant that's flexible and actually sticks things together (not like the usual silicon gasket stuff, which doesn't really glue anything together, just fills holes).

Genuine Ford and Vauxhall sealant are good, and reasonably cheap (Ford stuff comes in a handy little dispenser tube, whereas Vauxhall comes in a short mastic tube and needs a mastic gun). Failing that, Tigerseal is always good, but may make future dismantling a bit more challenging!