Would recommend a Grant Utility boiler as they are pretty rock solid. Avoid Firebird, Warmflow, Worcester or Grandee with the proverbial end of the stick. Have a look at the Grantuk website - loads on alternative energy systems and thermal stores too. Grant do multiple heat source systems designed so they can all be easily tied together.
If you're going for a total refurb, consider an underfloor heating system. (always plan out your bathroom first so you know where is safe to make holes in the floor later to mount WC, basin etc)