If you are logged in and you use Right Click to lock a position we now show you the approximate height at that point as well as the lat/lon and OSGB which were shown before in the red box.
It's clearly a 50m contour as it's a thick one but which? There's nothing that close to tell you so you're going to have to follow it around until you find a contour with a number on or count up or down from a spot height and work it out that way. But instead I've right clicked on the contour, the mouse location box has now locked (and is bordered in red instead of black) and the height is shown as approximately 451m. So this is clearly the 450m contour.
Clicking on the red cross next to the box will put me back into normal mouse position mode.
One caveat: this works well in general, with relatively small errors, but it's very poor on summits. This is a technical issue: we only have height data on a 50m x 50m grid and unless the summit happens to fall at a grid point all four heights around it will be lower so our algorithm, which works on a weighted average of the four nearest heights, will see it as lower.