Something I'll flag up here is that heat pumps need diverter actuator heads and not the more common mid-position devices.

I'm aware of the difference because after my heat pump system was installed, I noticed that, when set to heat the DHW, it was also warming up the heating system. The installer's boss came and diagnosed the problem: wrong type of actuator head as that's what the plumbing suppliers sell by default. It was an easy fix once the correct part had been purchased.

@editor -- stick your phone beneath the valve and take a photo of the label.

If you post the pic here, we can spend the rest of the day arguing over which part-number for the replacement head is the correct match for your valve.

Even if we narrow it down to a couple of possibilities, you could buy both for a lot less that the call-out fee you've just been quoted.
And, after all, two heads are better than none. 😉 

@transparent can't get my hand in to even take a photo. Laughable position.

And to compound issues we've woken up to a cold house two days in a row because of a "Major Defrost Failure" as the system delegated the defrost to the boiler which doesn't work.

The amount of shoddy installations has to stop!

Posted by: @editor

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can't get my hand in to even take a photo.

Have you got an inspection mirror?

This one from Amazon is £4.99

Posted by: @editor

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And to compound issues we've woken up to a cold house two days in a row

There's a book you need to read called Bodge Buster.

And there's also on online Forum where people discuss issues with heat-pumps and other energy-related problems.
If I find a link, I'll post it here!

The irony of this entire situation isn’t lost on me @transparent: if we’d had a properly installed, reliable heat pump from the start, there likely wouldn’t be a Renewable Heating Hub website (or books), at least not created by me.

I wish my younger self, sans heat pump, had had these resources when committing to renewable heating six years ago. 

I’m counting down the days until we can finally have our heat pump installation overhauled, which should be happening this spring. It’s been a long time coming, and I’m hopeful this marks the start of smoother, more efficient, times ahead.

You're contemplating staying cold until Spring?!

Is the reason that you can't get your hand to the valve because of the close proximity of the pipes,
or due to the wall to the left stopping you from extending your arm in deep enough?

Ie do you need a smaller hand or a longer arm?

@transparent, we weren’t planning to wait until April to address this issue. Instead, we’re considering gutting our entire setup in the new year and working with installers who actually know what they’re doing, and getting a decent heat pump with better controls in. This will be a set up to share with homeowners that shows best/better practices.

Regarding this specific issue, we finally got it sorted. What really frustrates me is tradespeople, like the boiler repair guy from last week, showing up, making assumptions without testing anything, and then recommending unnecessary work. He was in and out within 10 minutes, leaving us with no real answers.

Today, we called an electrician recommended by our neighbour. He made a genuine effort to reach the valve, but it’s in the most ridiculous and inaccessible spot. After a fair bit of cursing, he took a more methodical approach. Out came the Fluke tester, and within minutes, he confirmed the valve was functioning properly and there was no need for Cirque du Soleil contortions to get it out. From there, we systematically traced the system, following the electrical connections.

It turned out the cut-out switch on the boiler had been triggered by a jammed circulation pump. After some trial and error and careful troubleshooting, we got the system running as it should.

What annoys me most is that this should have been identified during the initial call-out by the boiler repair "expert". 

FWIW, here's my cylinder after the pipework was finished and before the pipe insulation was added. The 3-port valve is very accessible. It's possible that the cylinder designer gave thought to the layout of the various pipe connections to minimise the risk of inconvenient positions. Overall, the cylinder and pipework uses a space about 90cm wide and 85cm deep. That was more than I had expected.