The bulb doesn't seat very well at all and is able to move, albeit very slightly, in the holder. I'll try another manufacturer and see if that helps. Thanks for all the replies and happy new year!
H7 bulbs aren't a particularly snug fit in the lamp, but as mentioned previously it is important to get the tab in the right place. The low beam has the tab at the top and the high beam at the bottom. As you will have discovered there isn't a lot of room to work but with patience and care it can be done. Gently does it, the spring clip hinge is notoriously fragile. If you break that do a search, once again it is not the end of the world, there are dodges that have been tried and tested by resourceful Tossers!