@mike-patrick, glycol is a but pricey from what my installers told me, but it comes down to how much they poured in too, which I'm guessing they can't tell you in litre/gallon terms.

Tradesmen fees in the UK have never been "cheap", but I'm beginning to see a worrying trend emerge. Just last week, I spoke to an electrician about the prospect of him fitting our new Immersun power diverter. After a lot of deliberation on his part on the phone he "settled" on a fee of £450 + VAT to fit the unit. Madness.

It took me three hours yesterday to install myself (and yes, it's working). 

There's a guide to rates of pay for the various categories of tradesmen published here just last month.

https://householdquotes.co.uk/tradesmen-labourer-costs/

It's not as comprehensive as I'd like because the rates should also differ according to the geographical region of the country.
But it's a offers a starting point if you want to challenge a high bill.

Posted by: @mike-patrick

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There's £70 for glycol. This was to top up the system, not completely fill it. That seems expensive too. Any thoughts?

I checked at Stoves and Solar, who supply solar-thermal systems. Their glycol is £45 for 5-litres.

Choosing to buy from a different market outlet can be cheaper still.
Eg Tractorparts sell 5-litre Glycol for just under £31. Perhaps the agricultural sector uses more of the stuff.

I would expect an external system to use between 10-40% glycol in water.
Too much isn't a good idea because it has a lower thermal-capacity than the water.
In Devon I've dosed to 20%, which is satisfactory down to -8°C

Another site which provides Tradesmen's rates is the Building Sheriff.
He keeps the town free of cowboys, you see!

https://buildingsheriff.com/

You'll need to go down through the categories to find a typical price for each job.
And there's lots of jobs which aren't listed at all.

But Building Sheriff is compiled by tradesmen who actually understand the tasks which they're providing the costs for.

Thank you to all for your responses on this topic.

I've now had that further conversation with the firm, the outcome of which is £50 off the bill and a need to find a different firm for any future work!

The guy sounded genuinely upset that I should consider the invoice amount excessive, although he did lose his rag rather quickly which precluded a longer discussion. He said there was a lot of time put in trying to source the replacement tank (time which wasn't charged for in the invoice). But in response to my question about the 9 hours labour for fitting he sort of admitted that the engineer on his own would have done it in 6 hours (rather than the  4 and a half each for him and the mate). We clearly were not going to agree that the correct labour charge would be for those 6 hours.

It will be a nuisance to find another heating engineer but it's not that I've lost the one local firm that really understands ASHP installations. I'm still looking for them. I can deal direct with Grant for the annual checkup of the pump and the pressure cylinder.

Lesson for me: Always get a written quote up-front.

Keeping fingers crossed that there are no more leaks in the system (all of which I put down to poor original installation work). For now it is working as well as it has ever done in the last 6 years.

Mike