I've just paid BG £2350 to have a Worcester 34cdi, with Boiler IQ, fitted on 2 yrs 0%, 5yr warranty (looking into Worcester's promo to extend to 7), straight swap, little bit of pipework, new flue (extended to meet current regs), chemical clean and re-used mag. filter; BUT, that was with a 25% discount as I used to work there and I'm still friends with employees and got a 'friends & family' discount.
I did get three other quotes, though. One was over £3500 (local company, taking the proverbial, prob didn't want the job); another was about £2800 and I think the third was about the same and they were for lesser boilers.
I was expecting BG to be expensive, but wasn't expecting the local competition to be so too (or does it mean BG are getting cheaper?)
With BG's 2 yrs 0%, even without the 25% discount, I may well still have used them.
EDIT: I see you've mentioned it already. 
Look up the new
Boiler Plus regs. They stipulate new requirements including things like smart controllers/timers/thermostats. So it may be your existing controls don't meet them and they've quoted for new ones. BG tried to sting me for £250 for Hive saying my thermostat/timer wasn't good enough, but it was and after evidence from Honeywell, BG 'surveyor' removed it from the quote.
