Thanks a lot for your reply Damien.
Unfortunately I don't think I'll end up adding a plinth into the model, however I agree it is probably the best solution for the problem at the moment.
However the terrain editing tools you mentioned sound very interesting. That would be perfect for what I'm trying to do. Hmm... I'll have to look into this.
It'll be a good while yet until the stadium is finished yet (I've been working on it on and off about 2 years!) so hopefully the terrain editing software will be available when I finish the model.
Thanks again,
Jake