In my experience, although I like Combis, they should be for smaller houses, or smaller households. Even with thermostatic valves on the showers, they really don't supply enough for more than one shower operating at a time.
If you have a working airing cupboard with immersion cylinder, then simply replace your conventional boiler. In 18 years, the efficiency of these boilers has risen massively. you'll notice a good saving in oil.
If there's only two of you, then certainly empty the airing cupboard and have a combi instead.
Worcester Bosch, Vaillant... any of the top (more expensive makes). Steer well away from "budget" boilers. It'll end in tears ;o)