Well, it seems that evohome installers are very hard to find around here!
No one else from the Honeywell site has replied so I started Googling for plumbers around the area who worked with the kit. One guy from Beeston (all of 18 miles away) replied to say they didn't cover my area. I eventually got a reply from the Mclean Electrical over in Grantham who said they'd be happy to quote for the work, so we'll see how that goes.