Thanks for the information, chaps!
I should have specified that I was only interested in country-wide digital mapping. Indeed, I was not aware that there are websites that just cover specific regions.
Has anyone experience of using Quo? The software is free but then you have to buy the maps which appear to be cheap. I'm not sure how this would work out if, like me, you are likely to want maps anywhere from Cornwall to Caithness.
My annual subscription to Grough costs approximately £20.00 for which I get a service that covers virtually all my requirements. I've never yet exceeded my print-out allowance and, in any case, I can buy additional credits for a very modest price.
I also use Wheresthepath? for the Google Earth aerial imagery linked to the OS map. This can be helpful, for example, when I need to establish that there still is a bridge over a particular stream. Also, as I'm interested in landscape history, I like comparing the modern map against the pre-war OS maps available on the site. I send a donation from time to time to express my appreciation. Which reminds me...
Hugh