Hi everybody..
A bit of a puzzler that I'm racking my brains over....

I'll set the scene:
My mums heating system (gravity fed with a header tank in the roof space) has three or four rads upstairs , 3 normal and 1 slightly taller towel rad in the bathroom not hugely taller , 4 ft perhaps?

Anyway: It only gets hot at the bottom 1/3 - the usual problem you'd think , bleed it ?
So I open the screw let out a tiny bit of pressurised air, heat rises a bit up the rad (one bar) but no further air (or water)

So I remove the complete nut that houses the bleed screw at top of rad, No sign of water or air 'rush'?

Soooo thinking laterally, I start to fill the rad with a kettle!, several kettles and although the rad is now getting hot close to top , the water never reaches to top of rad ???

and then I hear running water outside the house, on inspection the roof space overflow pipe is letting water out , if I stop with the kettle filling the roofspace overflow eventually stops , so this is kettle water leaving the building!!

What's wrong ?

and how the heck is the rad not filling up when the overflow is obviously above the rad in question?