Thats good news
Take the V5 with you....altho vx dealer should be able to tell how old it is. But we all know what muppets they can be sometimes!
Leave the service book at home.....just in case they do take a peek at it.....and then say its not been vx serviced all its life (assuming it hasnt).
Reason for this is.....the yearly anti corrosion inspection is free....and therefore can be argued that vx dealer should have done it when they serviced the car.....so if the service book isnt upto date with vx dealer stamps.....then they could argue it hasnt been at a vx dealer for the last 'x' years so inspection couldnt have been done.
And assuming the rust isnt a stone chip thats been left to rust....vx dealer will ask for authorisation from vx to do the rust and in a couple of weeks the paint shop at vx dealer will be ringing you to book it in to be sorted.
PS do you get your mum to do/find everything??