, you need a conventional boiler. It will be condensing which just means extra efficiency by using the heatfrom the flue to preheat the water. Best brands are valiant and Worcester, but baxi potterton do some great ones that are fairly cheap
Check the kilowatt rating of the boiler you have now and if you are not adding more radiators get something similar or just a bit more powerful.if your current boilers is 15kw then there's no point buying a new 30kw.
Toby, Last thing, will I notice a significant drop in my gas usage using a new boiler (bearing in mind the technology of my knackered one!) or would I have to go Combi type boiler to notice a saving? It has to be said I have an invalid in the house so the emphasis is on maintaining a comfortable temperature rather than high volumes of hot water 24/7 and the like. How long is a piece of string? Regards