If it was me mate, i would have went mental at them!!
However, i would say the best thing you can do is write a letter of complaint to VW UK (copy in the dealer principle of SKF Oldham) explaining exactly what happened and how you were treated after you went in to complain and seek a valid explanation.
Points in the complaint to cover would obviously be the unauthorized use of your vehicle and the shocking level of customer service (being on hold for 14 mins is a p155 take). I wouldn't just seek reimbursement for the fuel, i would demand the whole cost of the service to be refunded. Some folks might say this is being greedy but if they thought they could use your car as a runabout, who's to say they didn't rag it?
Instances of of such bad levels of customer service needs to be highlighted and reported, otherwise it will just keep happening.
More importantly, the unauthorised use of your car needs reporting to the highest level. There was a recent case of the 2 kwik fit mechanics who took a customers Porsche for a spin (without the owners consent) and tragically knocked over and killed a little boy. Ok, its a bit different in your case but if they are driving your car without your consent, doesn't that void their insurance cover?
I hope this helps and certainly hope you don't just accept the way you have and your car have been treated. You pay the premium of using a dealership for a reason!! For me, the trust with the dealership has been lost and i would never use them again.
Thanks
Rich