My sister had to replace hers last winter. British Gas quoted about £3K. She found a private plumber who did it for about £1000. A decent boiler costs about £800, then labour, which shouldn't be too much for a straight swap.
I'm seeing my sis on Wednesday so I'll ask he for his details.
If you get a condensing boiler, which is what you'll probably get these days, if the condensate drain goes outside, make sure you ask the plumber to insulate it, as they freeze up easily in the winter and your boiler will cut out when you least want it to.