Ditch the salus products.
Get an independent company out to price installing a Honeywell, or even vaillant own thermostat.
Tell the company that installed your boiler you want them to pay for it, hopefully they will agree, likely be in the form of reimbursement.
Before accepting any form of imbursement you need to get the gas Safe cert off then and the vaillant warranty registration.
That will likely end the relationship between you and your current installing company, but to be honest I don't think you are going to get any further with them.
When your newly chosen installer comes to install the new thermostat, ask them to look into the pressure loss issues.
This will cost you more money, but you clearly aren't getting anywhere with the current company