Baxi solo 250pf?

Hi Every one, i would like some advice please i have the Baxi solo 250pf boiler my problem is i have 18 rads quite a few doubles and a couple were turned off when we moved in its a 4 bedroom detached house so I'm wondering if the boiler 15 KW is big enough to cope as in this cold spell the rads don't get that hot and its that cold we need to have gas fire on as well to try and keep warm i have been told to put sentinel X 400 in the rads to clear any sludge etc but I'm thinking of changing boiler to a vaillant ecotec plus 418 any suggestions or advice would be greatly appreciated