StuartYour service levels apparently suck. Over here in AU our roadside assist crews from the Motorist Associations change them on the spot.. Never been an issue. As long as you/they have a triple square bit on them. Buy one and throw it in the glove box.
I know Shropshire's a funny place but FFS, any good garage can do the job - it's not bloody rocket science!
It will take them 30 minutes tops.
Changing the T1, T2, T3 and T4 battery, including both batteries in the V10 - myTreg forums
Take care to observe the battery connection sequence. I can't recall what the manual says right now, but I know when I did it out of order the car was very unhappy... turning the ignition switch would do nothing and it trapped the key in the lock!