As a gas boiler engineer, it sounds as if either the pump has stuck or the feeder tank in your attic has ran out of water ( ie the ballcock isn't letting water into the feeder tank.
However, if you were intending to change your boiler, I would have no hesitation in changing to a Combi system, for 2 main reasons - 1) the heating system will heat up quicker and warmer and 2) much more economical to run.
After 2 years of researching into High Eficiency boilers the only one that I would recommend is the Maxol Supacombi HE28 - well built with copper and brass fittings, NOT the plastic rubbish that most manufacturers and now using. The will mean that the boiler will probably last 20years not approx 8-10 years as with the plastic crap.
cheers
Ian