My plumber quoted £2,500 incl VAT (parts and labour) to replace my
conventional boiler with a 28KWATTS Worcester Combi Condensed boiler.
The boiler costs about £750-800 incl VAT. Is the labour cost too
expensive?

A couple of years ago we had a combi replaced with a new combi - labour
was £400 in West London.
Ask him what's going to take 4-5 days work - is he replacing/re-routing
alot of pipework?

Personally, I would get a bigger rated combi - at least 35kW - it
doesn't cost much more and the flow rate is much better.

Charlie