Old Ordnance Survey maps keyed to Google Maps 🌍

When researching mills and mill sites, maps are a key resource, and I've already tied this site closely with the mapping services provided by Google maps - offering an aerial view of each windmill's location via the service.

Another service that looks like it will be very useful is Old Ordnance Survey Maps which offers OS maps from around 1925 - 1945 via the Google Maps interface. Coverage of the UK is patchy, and the site is sometimes rather slow, but these are minor distractions.