Its all good. We've all been there at one time or another.

At least you picked one that's easy to work on and has a good enthusiast community behind it.

 

Hopefully won't be a big issue for you. If you have not changed the oil already, go drain it and see how it looks. Even if you have changed it, would be worth a shot at seeing what it looks like. To me though, it does sound like exhaust gases are pressurizing your coolant. Just shouldn't be that much to blow off caps or other parts.
BTW... give you sig a quick change. No such animal as a "Range Rover Disco". I'm sure you could make one, but you have a Land Rover Disco