baxi solo 250pf gas boiler advice?

I have a baxi solo 250pf gas boiler that was fitted in a 4 bed detached property before we purchased it with 18 rads & a couple are doubles it doesnt seem to warm the rads very well in this cold weather nor do we get a lot of hot bath water this time of year from a tank that stores constant hot water for heating & domestic supply it seems a rather small boiler for the amount of rads and size of house please could you advise me on a more energy efficient A rated higher output boiler.