Thanks for all the replies guys. Just as a tailpiece to this I ended up with 7 Hornby wagons, mainly from ebay, and then some unpainted Dapol ones came up cheap locally and as I intended to respray the things anyway I went for them as well.
Others have pointed out the cosmetic differences between the two, but the difference in running quality was quite staggering. The 7 Hornby wagons, with a GUV attached at the rear, ran perfectly, whereas the same length train of Dapol wagons would scarcely run a few feet without something falling off or coming uncoupled. Having checked the wheels were turning freely and the B to Bs were ok I realised the Horny wagons were noticeably heavier. About 15 grams per wagon in fact. So I added about 12 grams of lead flashing under the Dapol wagons (taking the things apart and putting the weight in the tanks would have been a more elegant solution - but life's too short!) and they now run perfectly. Credit to Hornby I think for recognising the problem and putting, as far as I can tell, some weight in the tanks.
Thanks again to everyone for your help.
Bill