Good news! Autoglass came out today and a very pleasant fitter spent two and a half hours very carefully removing the old screen, cleaning everything up and fitting a new Pilkington branded replacement, complete with rain sensor. My wipers work as they should again.
He cleared up a bit of a mystery - the other one hadn't cracked due to a stone chip, rather the previous fitter had fitted it badly and the left hand plastic lug at the bottom of the screen had put a point load on the screen and cracked it. Sadly I don't have any of the service history from previous owners so I don't know who fitted it.
The only things outstanding now are the DAB aerial (which, thanks to another member here shouldn't be to much of a problem to sort) and also I think the fitter disturbed some wiring in the headlining as there's a rattle coming from above the interior light assembly. When I get a moment I'll take it apart and see what's going on.
Thanks everyone for your input and I hope Dawn's car is sorted soon
