After trying a few different methods I can finally say the only thing that really worked was sugru self setting rubber, even after a heavy downpour or leaving the bike out all night in the rain, the next day you can be confident of nothing seeping through and giving you a soggy behind.
Its even heat and cold resistant to 180C / -50C, so no melting in the hot sun or cracking on a winter morning.
Application tips : Dont remove the hump and apply a bead of sugru around the perimeter of it, this is what I did initially but it didnt quite work, what to do is make sure the hump is screwed in as tight as possible then with a finger, smear the sugru into the edge like you would if you were grouting some tiles, this ensures a completely watertight seal.
It really makes a difference knowing youre not going to end up with a wet arse.
http://sugru.com/about