Took her in this morning and she was super well behaved!
The vet examined her, took her temperature and weighed her again and everything was fine. Then he went into extreme detail about everything on the consent form - I was quite impressed, usually I just leave the cat with the receptionist, sign the form and leave, so things must have changed in the last 12 years, or this practice is really good. It's made me feel a lot calmer about it all anyway.
She's also going to have a blood test prior to the treatment, to ensure there are no underlying conditions which may cause problems when she has the anaesthetic (of course, this costs extra but given her age I thought it was a good idea).
So just waiting for the phone call now, which should be around 2pm.