Ask octopus energy, in writing, addressed to “in house counsel” and cc-ed to their engineering director (or other “installer” contact) something like this:
"I have been told that a installing a monitoring solution for my heat pump, a monitoring solution which octopus themselves use, on my a heat pump which octopus by their attempted remedial actions acknowledge has had a number of operational and installation defects, would invalidate the warranty on the installation.
I see no such clauses in our agreed terms and conditions.
Can you please confirm in writing that in house counsel also state that this would invalidate octopus warranty on my heat pump installation, that acting to prevent scrutiny of the octopus installation is compatible with the terms octopus agree to under MCS, and that you believe this does not constitute an unfair clause in a consumer contract.
I expect a written confirmation within 14 days. Should you not provide written confirmation within 14 days I shall take it as read that octopus in house counsel do not state that installing a monitoring system, as used by octopus, on my own installation, will void my warranty and in fact has no affect on my warranty and octopus ongoing efforts to complete the installation to an adequate standard."
That’s a big “please put you neck on the block, legal, for what the installer team are saying to prevent me checking their homework.”
The hope is that legal don’t want the hassle of taking a formal position and therefore allow you, by omission, to install the widget.
Expect the relationship to deteriorate though; and Ultima to have to pay somebody else to finish the job (replace that spring shut diverter by the sounds of it) and invoice octopus for costs, who will no doubt try to cover their backsides by claiming not to be experts in the field and that Daikin said this valve would be fine even with an excessively high dP / high flow rate etc.
I do hope that anybody who still thinks the sun shines out of these guys backsides are taking lessons from their backside covering responses when their dice rolls don’t always land a six!