Combis can run that amount of rads easy, its the hot water delivery

that lets them down, basically there crap

There fine for small houses or flats, but for larger houses I would never fit one I'm afraid.

Rip the back boiler out, and if funds allow go for a pressurised system,

with an unvented hot water cylinder, and heat only boiler.

I always fit Baxi/Potterton boilers but any of the big names like;

Worcester or Gloworm you can't really go wrong.

Cheers