afaik i don't think you can use normal tickets on it. Me and FGWfan are considering getting it up together if I can come so he'd be the man to ask, he obviously knows as he said we can both go for £20 somehow.
I know that Rail Magazine are doing a special 'buy one get one free' offer, but that's only for 2 people.
I can get there for £17.50 Rtn, but I dont mind paying a tad extra for a more entertaining journey...
Apex doesn't have the same rules of 'validity' as other tickets. Along with SuperAdvance and other advance purchase tickets, it's only for use on the train specified on the ticket (although in the case of connecting local trains, any can be used, if applicable).
So it is more correct to say Apexes aren't available for this train, rather than not valid.