Thanks everyone for the replies - they really were appreciated.
In the end I went for Westmalle triple and double (absolutely beautiful beer - some of the finest I've tasted). I also went for Duvel which despite being readily available in the UK was much, much cheaper in Belgium (and even cheaper in France).
Unfortunately I found a Leffe offer which was too good to refuse. The 25cl bottles worked out at less than 65p a litre (with the free bottles) so I packed the small 1.2 litre car up with more than a good few. This was of course, in hindsight, a mistake because there wasn't room or weight for much more. Still, I also bought some Gauloise beers (wish I had bought some more - very nice) and Blanche des Honnelles which was absolutely amazing (all gone now).
The best, imvho, is the Westmalle. I don't think it's available in the UK. It really is a nice beer. The triple tastes like a triple but is still worryingly easy to drink and the double surpasses anything I've tasted recently.
I'm hoping to go again later in the year with more knowledge and the benefit of hindsight. The Leffe is at least drinkable and the 25cl bottles do taste much better than the 75cl bottles (of that nobody can convince me otherwise!)
Thanks again.
Gareth.