Dillsue

Assuming your roof is in fair condition and doesn't need reroofing anyway, there's lots of videos on YouTube showing roof hook installation on slate roofs without a complete strip and relay. I'd be tempted to use an experienced slate roofer to fit the hooks rather than a PV installer, unless they employ a good roofer themselves. Let the PV installer fit rails, panels, wiring etc

Totally agree on the MCS pricing but just giving you the heads up on export constraints. I beleive Octopus also want to see building control sign off so there's another cost to add to the Octopus fee.

Octopus do accept self installed systems but there's hoops to jump through and £ to pay! Have a search on here for discussion but I think consensus seems to be it's typically not worthwhile to bag an export tariff that can be withdrawn at any time.

You mention profit from export. Currently you generally need an MCS certified installer to get paid for export so you may want to get your installer involved in the design/spec to get their buy in.

You don't need to hunt for the rafters as there will be screws/nails in the battens which hold the batten onto the rafters. These will show you the position of the rafters for you to fit the roof hooks. I'd check your battens and slates have been fitted correctly as there's usually space between the top of the felt and underside of the battens to allow ventilation/water run off. Im no roofer but AFAIK the felt usually sags between rafters to form this space or if there's boards below the felt, counter battens are used. If that needs changing get it done before you fit the panels and get the roof hooks fitted while the slates are being laid.

If you're running weather compensation on the heat pump and it's been set up correctly, why do you need Wiser TRVs connected to the heat pump?? The WC should take care of individual room temps via balanced flow through the rads. Connecting the Wiser system to the boiler will control the house temp when the HP tells the boiler to run. If there's a valid reason for the HP to have a signal from the Wiser system then install a relay, as Sharpener suggested. With a 2 pole relay in place the Wiser system will switch the relay and separate contacts from the relay will separately switch the HP and boiler enable/call for heat signals. What has your installer said about the boiler overheating the house??

If you're not sure on the config you want/need, get yourself a secondhand inverter off ebay to get the panels up and running. No pressure to make uninformed decisions and fair chance youll be able to sell it again when you know what you want.

A concept often missed in our quick/cheap fix world

Shouldn't be a problem with the right sized pipes. Remember that your not pumping up to the loft but circulating up there and back down again. The flow up to the loft will be working against gravity but the return back down to ground floor will be working with gravity and the two gravitational "resistances" cancel each other out. All your circulation pump needs to overcome is the restistance of the pipe work and fittings

I know for a fact that at least one DNO has accepted self installed and N/A for qualifications on both G98 and G99 commissioning docs. G98 is a notification and I truly belive any DNO will appreciate receiving them regardless of the installers details

That's good going. I played email ping pong for a few days, then they asked for the detailed calcs which I sent them and they never replied.

Collecting parts at the moment. Once it warms up will swap/fit new radiators and then get the heat pump fitted. We're having a hybrid system with WBS and LPG boiler as backups so aiming to have a slightly undersized HP with the WBS topping the heat up on the very few freezing days we have.

My experience with Cool Energy is they massively oversized our heat pump suggesting a 16kw pump for a 6.5kw heat loss. When I gave them the detailed heat loss calcs for comment, they never replied. There's a recent thread on here where they offered a 10kw pump for a 3kw heat loss. It's likely that sort of oversizing and attendendant high running costs are leading to the bad publicity you're reading.

I think the dilemma is whether you can put self installed which Roger wants to do

It's a G98 notification so export is already happening and an "installer" has already done the work. In theory the only legitimate answers are Building Control or a part P spark but some DNOs do accept self installed;)