We just paid £2.600 to have same boiler Worcester Greenstar 28 installed, with 5 replacement radiators, (we had two newish ones already) a new stopcock, so I would have thought just to have boiler installed was a bit pricey. Have you tried a few more quotes? He also removed old cylinder in bedroom, and cut off old lead pipes etc, he also capped a gas fire in lounge for us too. He was 4 days doing the job, with himself and another chap. We are in the north west of the UK. Perhaps that makes a difference to pricing.