My '06 petrol vRS is on a variable service schedule, according to the schedule. It's got the QG1 on the sticker.

Following on from this post :

http://briskoda.net/forums/topic/150225-fixed-vs-variable-servicing-on-petrol-vrs/

I think I should switch to fixed interval as I'm only doing about 25 miles a day to and from work, and a bit of driving about at the weekend.

Do I have to go to my local dealer and pay them to service the car and swap it over at the time ?

Or can I go to a local garage for a service, and have them change the interval while they're at it ?

Just reset the indicator as described in your handbook when the car is serviced. Tat will set it to fixed intervals.