I have discovered Bing Maps, an online mapping service. So far I’ve added three short routes to My Places. Hover over the arrow next to Road and a menu appears with Ordnance Survey Map as a choice. Clicking on this and zooming in produces first the 1/50,000 scale map (Landranger) and then then 1/25,000 scale map (Explorer).
Sign in with a Microsoft Account and then you can mark out routes and save them to My Places. The site takes a bit of getting used to but it looks promising and useful. It measures the distance of plotted routes which could save the embarrassment when a walk turns out much longer than expected.