Not always neccessary to replace complete boiler - depends on the general condition of boiler , ie how often it is serviced etc. If the boiler was in not to bad condition then replacing flue is probably the cheapest option. We replace 20yrs + flues regularly because the boilers are in reasonable condition for age.
However if you are going to replace boiler then you could go down to B&Q and get yourself a Heatline combi for about £400. Being a service engineer for Heatline (plus other manufacturers) these are very good boilers for the price - the parts used are exactly the same as more expensive boilers ie Honeywell gas valves, Grundfoss pumps.
The difference is that they are put together cheaply - in Turkey and are owned by Valliant.
For reasonable mid range priced boilers, we use Sime. Have been using them for about 12 years now and havent had too much trouble with them. Also the Halstead boilers are not too bad a bit more expensive though , but like everything else in life are easy too repair if you know what you are doing.
Cheers
Ian