Quite agree. At my last place (identikit house to the others on the estate) the downstairs cloakroom was bleddy freezing in all the houses. A neighbour *sorted* his by sticking in a much larger radiator. I checked things out first and found:

a) ceiling was just plasterboard - no insulation
b) ceiling light fitting let cold air in
c) cistern overflow through wall let in a draught - the pipe wasn't sealed plus you get cold air into the cistern. Put in a smaller syphon which overflowed into the pedestal rather than through the wall.
d) waste outlet pipes for loo and sink went into the floor. Neither were sealed around their circumference.
e) electric meter was also on the wall. The wires went through the outside wall to the meter in the 'exterior cupboard with no sealing. Expanding foam to seal this.
f) single glazed until we had the whole house double glazed.

Probably other things as well .... but the upshot of it was that some sealant fixed most of it plus some loft-type insulation above the ceiling. Nothing complicated. And it was dead toasty afterwards!

The other top tip though is to search out draughts in the current cold weather. Much easier to find, even if you fix the later in the year.

For rads. fit TRVs to all of them (except the ones in the room with the room stat.) and if they're quite old take them off the wall and flush them out with water. Best leave this until the summer! When I did mine (some were blocked) they were much hotter afterwards.

Dave