The boiler itself will probably cost around �600, or so I was informed by a kitchen fitter friend of mine when we paid �1500 to get ours replaced last year, it is the time and labour that adds up the cost. If you know a plumber you probably would be able to get it quite reasonable. Offer to pay him cash and he may give you discount. Good luck.
Also go for a good make, there are certain boilers that are available from DIY warehouses that are not too well thought of by the trade. They are cheap to buy, but spares and after sales can suffer. Ask the advice of the plumbers, as to what they would fit in their own homes.