Clearly I was tempting fate by blogging about how amazing Autoglass is before they had actually fixed my windscreen. Because they didn't show up. And then there followed predicatable call centre fun and games ('we don't have any record of your booking, sir'; 'ah, yes we do, I don't know what happened, the local branch will call you back'; 'the branch didn't call you back? They will definitely call you back between 0800 and 1000 on Monday').
They never called back.
So I called them just now, and got pretty similar story: the local branch will call you back.
Just shows that good service is all about processes, if the process is well thought out and works, then customer gets great service. If something goes wrong, whole process can go awry. I think what's gone wrong here is that we changed the original booking, and that was done with the local branch, and local branch just somehow lost/deleted/cancelled the booking (instead of changing it).
Although why the main call centre can't just force a new booking into the system and figure out the detail later I don't know.