So I turns up and the conversation goes something like this:
"You're Mr Evans?"
"That's me"
(Stern look) "Where were you yesterday then?"
(Confused look) "Yesterday?"
"Yes, you were booked in for yesterday."
"I'm sorry, I was sure it was Friday..."
"No... (etc)"
The mechanic then proceeded to show me where he'd booked me in the diary for Thursday 5th.
"I'm sorry, I really thought it was Friday 6th."
"Well, we're up to our eyes in it, but I'll see what we can do."
So I went away feeling "what a bad start" - as this was the first time I'd taken a car here

Later on, back at work I had a rummage around and found the scrap of paper that I'd written on while booking with him on the phone, and checking directions to the workshop a couple of weeks ago. It plainly said "Friday 6th January"
Aha!

So during my morning break, to prove I wasn't going crazy, I walked down to the garage brandishing said piece of paper.
"Oh... I don't know how I did that...", he said.
That redressed the balance!

While I was there, he informed me that one of the rear radius arms was going... which was the source of the *crack* noise I noticed when I got in the car in the mornings recently... Ho hum. But he also says the thermostat is working fine.
Mark.