I'm pleased to be able to announce that we are now serving Ordnance Survey 1:50,000 (LandRanger) tiles as well as 1:25,000 (Explorer/Outdoor Leisure) on our mapping so long as you are logged in (if you're not you will continue to see our OpenMaps/BigV tiles.
For large scale maps, so 1:100,000 and above, we continue to use OpenMaps/BigV tiles as they provide the best mapping compatible with Ordnance Survey tiles.
N.B. there is currently a bug which will only affect you if you use the "+/-" zoom tools rather than the mouse scroll wheel to zoom. If you do then this switch between tile servers does not happen. I hope to resolve that very soon.
I'm pleased to report that the "+/-" zoom bug is now fixed.