I had to replace mine recently - it was a Combi which makes things simpler, but I ended up getting it through
https://www.boxt.co.uk as it was an emergency. Delivered next morning direct from Worcester Bosch along with an engineer to install it. They give you a guaranteed price (I think) through the website.
The engineer wasn't the best, and I'm sure I would have had a better experience from a good local installer, but for speed and for a fixed cost, it worked for me. They'll also put it on the never-never for you.