I used this email address back in early 2019 so hopefully it's still valid:
[email protected]
I was charged just under £95 for the engineer (just checked the invoice). I can't remember whether they charged me for the parts on top of that or not and the invoice doesn't list it. But I recall the whole thing being quite reasonable at the time. I had to supply them with the serial number from the chair. Mine was on a label under the seat. Then they booked an engineer for a couple of weeks later. He took about an hour to strip it all down and fix it. Then they sent me the invoice a few days later. Unless they have changed the process now that's why I think they are more used to dealing with companies rather than Joe Public. But they were very helpful.
I think they could only send an engineer because I was near London where they had a service centre (they might have others). Or I could have fitted the parts myself or even taken my chair to them.