The reason I think 1300 ex VAT for an install/replace is a bit steep is that I've had 3 new boilers fitted [in Holland] in the last couple of years which took around 5 hours max each. The company doing this London job were asking 70 quid an hour which I commented to my housemate who arranged it seemed cheap. But 1300 ex VAT for 5 hours work would be 260 per hour.
Emergency rates differ to "normal" rates. If you need a plumber/GSR engineer for an hour in London, don't be surprised if you pay upwards of £150/hour. That doesn't mean that the going rate is £1200/day in non-emergency situations. You can do a quick search for my previous post to see what I found the going rate was near me, but others on here disagreed. Best thing to do, as said already by another member, is ring 3 different people for quotes and this will get you in the right ballpark to see if anyone is charging an extortionate amount.
And OP - you have come onto a forum complaining about the price you are being quoted by a GSR engineer. This forum is frequented by GSR engineers. They're hardly going to talk down their own trade and say you're being overquoted. I can understand why they get frustrated when people try to negotiate or barter down the price, but that's consumerism for you. If you don't agree with it, probably better not to offer your services to the public - it's unlikely you'll ever get consumers to pay a price without questioning what it is comprised of.