You've answered your own question really. Yes it could pressurise the water if leaking into the water. Think Saint on here had very similar issues a while back.
I've plenty of off road experience so not overly concerned (hope I don't regret writing that :p) just the sill tanks I'm a bit bothered about. Will probably take the front dam off, bolt a couple of jate rings on and chuck some ats on anyway :)
Oh, and I've got no recovery points on the front, a front air dam and 205 road tyres on it too. What's the worst could happen? :D
It's a Range Rover classic, no problem. :D
I might want to go laning on the Saturday but have under sill gas tanks and was wondering how rough, if at all, it might be? Seem to remember it being fairly easy going from what everybody said last year?