Buy a Bosch Quigo from Amazon or somewhere.
Self Levelling Laser, paints a cross-on a wall, acrow, floor, lintel, door, shelf, tiling, wallpaper, sockets, wood panelling, anything. Suspect it is not good enough outdoors, but it is absolutely wonderful otherwise.

A good cheap spirit level is a Stabila 24", 0.5mm/m accuracy, german, about £12 delivered if you shop around (sometimes better).

The Quigo is a wonderful tool though.