I agree that they're not the best firefronts. However I recently did some digging attempting to find a replacement for someone and it appears that all firefronts for the Glow-worm BBU's are no longer available. A word of warning is don't even attempt to try and fit a different manufacturer's (Baxi) as these won't fit correctly and will spill fumes into the room. Fatal! Any good CORGI registered engineer should refuse to do this work anyway. Do NOT be tempted to do any work yourself.
Unless you can obtain a suitable firefront secondhand I'd advise to save your money and eventually get a condensing boiler (90% efficient against the BBU's 65%). You can then install a seperate gas fire or even an electric fire. The latter are a lot safer and should only have to be used as a back up heat for breakdowns.
Stephen
November 2007