Condensing boilers are more energy efficient, Combination boilers are one type of condensing boiler but all new domestic boilers in the UK are now condensing, and the efficiency difference between combi's, system boilers and heat-only boilers (your present one is probably a heat-only) is negligible. Just make sure your installer fits a SEDBUK Band A boiler and you'll be fine. Expect to pay a bit more for a decent one, I wouldn't recommend going for the cheapest you can find, there's lots of info on these forums about which makes are good and which are not so good.
Combi boilers are not the be-all and end-all by any means - lower-powered ones tend to give very poor hot water flow rates, higher-powered ones improve this but are not necessarily the answer as they may be over-rated for your heating system. Also if it breaks down then you will have no hot water until it's fixed, as there's generally no storage (unless you go for a storage combi, which works slightly differently).
EDIT: Having said all that, as you live in a small flat, a combi would almost certainly be your best option, as it's unlikely you'll have a high hot water demand