I don't think there's anything wrong with their system Trev. It's almost certainly a case of a combination of peak travelling season, short notice and 4 seats required. At this stage, 4 heavily discounted seats aren't going to be available.

I haven't tried it but you'll probably find the odd date with a slightly lower price if you search for 1 ticket with flexible dates. It still won't be cheap though because it's peak season. I'll bet Virgin is a very similar price (if not higher).