OP, don't forget whoever quotes you may have to allow for complete replacement of all piping installed by previous cowboy - so will cost more than a simple boiler swap.
I see Muggles beat me to it, but I was going to suggest asking Dan for a quote. It will be well worth an 8 week wait to get the job done properly - and that is no more than you deserve.
I doubt the quote will be cheap, Dan is a craftsman. Look at the gallery of installations on Jennings web page. Poetry in pipework!
Whatever you decide, we hope it all works out for you.