Hi Murdo, thanks for bringing this to our attention, it appears that there is an issue if you zoom out a long way on the OSGB map then switch to another base map as it takes you pretty much anywhere but where you originally were looking at, we will look at getting a fix for this case, but its a little bit of an edge case and I can't say when we will get around to fixing that.
With regards to the data not appearing, we have a maximum zoom level applied to the different levels of data (10km, 2km, 1km and 100m) and these will stop mapping if you zoom out to a point where the data is no longer usefully viewable anymore, This is denoted by the entry on the UI becoming transparent and greyed out.
With partial displays and dots but no outlines issues, I would need screen captures to see what you mean as we have recently released an update to greatly improve the performance of the mapping, but I have not experienced these issues at all, in fact apart from your issue with the base maps I haven't encountered any issues since that update was released.
Matt
Edit: Having looked a bit more closely, it looks like Openlayers is swapping the lat/long values of the current position and dumping you around the coast of Nigeria every time you zoom out too far from the OS base map
Matt Debont
Application Developer
Joint Nature Conservation Committee, Monkstone House, City Road, Peterborough PE1 1JY, UK