Andrew, a picture of a two port and a three port valve just complicates what should be standard fit. Can you speculate how the system is wired so a would be controls installer can speculate what the Labour cost is liable to be
What Is your Evohome list of components to enable your system configuration to work, will you need the installer to assist you with what to get?
Component interconnect for Evohome, while may look simple, often needs careful planning to ensure good signal strength- is you property, in your estimation, going to effect this all important signal strength?
I do not have a problem with customer supplying the gear as long as they know any component issues will need resolving by the supplier (you) as I would only be supplying the Labour. Warranty again would be for fitting the components, so subsequent breakdown and replacement would attract additional charges unless the fitting was incorrect. I have just finished replacing a boiler supplied by the customer. I know if the boiler was to fail, in warranty repair will be carried out by the makers. With any control system, doubt the makers will come out.