Well, inspired by all the suggestions, I decided that Polaris nexus was a good bet and found that my LBS was the local dealer. I went along to their newly revamped showroom, expanded to two floors, only to find no Nexus in stock of any size at all. Not a great surprise, I suppose.
They offered to order a pair for me and replace it if the size was wrong, which seemed fine until I asked what if when they arrived, I found the cut was nothing like the advert, as per the previous three pairs of shorts I had ordered online in the last twelve months, and I didn't want them. Er, no, I would have to pay for them - they would only change if the size was wrong. I pointed out that this completely negated buying locally as I might as well risk less money buying them cheaper online or just stayed at home. Anyway I gave up and left.
Went round several sports shops and ended up at Blacks, where a weatherbeaten Koga rather like Mark Beaumont's was propped in the doorway (which I assumed was an exhibit but it turned out to not to be). I ended up with a pair of Craghoppers Nosi Cargo shorts, which although a bit long and flappy when standing up, provide a really good close fit on the bike. Slim seams and pockets that don't stick out. I've just been out for a quick 25 miles this afternoon (where did all the flies come from?) and they are fine.
I never would have bought them based on their online picture because they look nothing like that on me.
http://www.craghoppers.com/webapp/wcs/stores/servlet/Product2_13552_-1_25754_73935_11051_25287
John