We've put the radiator under the window in a small double room but have
now decided that the only way to put a double bed in the room will mean
that the head of the bed is right alongside the radiator.

No problem. Rememeber that the vast majority of a radiator's heat comes out
as convection straight up. If you don't block the airflow up and don't block
the cold air at the bottom, a radiator will work just fine even if blocked
in front.

If it is a child's room, consider fitting a low surface temp rad, or a
radiator cover (which is much the same thing, but less efficient).

Christian.