I would say £300-£400 depending on condition. They are about £625 new I think.
The Diatal 6x42 is a classic and a benchmark against which others are judged. If it is in good nick and you like the reticle, go for it!
But if you are looking at the S&B at £500, would take a look at the Kahles 8x50 that MacLeod's have with the illuminated reticle. A cracking scope...