In reply to JimmyKay:
Just thought I'd post an update to this thread in case anyone is ever in the same predicament.
We tried everywhere and they all seemed to stock the new white guidebook which is excellent for circuits but not much else.
Anyway after almost giving up on the 7+8's guide we went back to tourist information and they told us about an English bookshop just round the corner. They stocked it! In fact they had loads of copies of 5+6 and 7+8's! The lady who owned it was mega nice (and obviously has the right business model!)
The shop is called Reelbooks and is just round the corner from the mansion in the centre.