Stupid Mistake!

Last week I had a hire van for 2 days as I actually had some work (thin on the ground at the mo). After doing a 14 hour day in my tireness and stupidness I put ?40 of petrol into the van without thinking and drove a couple of miles before thinking something wasn't right then realised I'd put the wrong fuel in. not good. Yes have a good laugh what a twat I am!!! But it gets worse. I looked at the paper work in the cab as who to call, there was the office number and a mobile number and a merc breakdown number (sprinter van). As it was nearly midnight i decided not call the office as it was out of hours so rang the mobile, no answer so rang the breakdown recovery. Van got taken away, and I got dropped home. Next day called the van hire company and they told me I should have rang the office number as it was diverted to a mobile and the mobile I called was their boss's and he was on holiday hence no answer. Then proceeded to tell me that due to me calling mercedes breakdown it will be expensive. ?620 expensive to be precise!!! The van hire company have already taken that out my bank without me even seeing a bill which they say is in the post. Also they claim that its not covered by insurance.
Is it worth me trying to fight for some of this money back? I know i made a mistake and I'm prepared to pay but not ?620 due to merc sorting it out when the paper work in the van shloud have stated that the office number is 24 hours and there people would have done it for half that, also no diesel stickers on the fuel flap or key fob. They aslo claim that me signing the paper work on the original lease form means that they are entiltled to take the money out of my account without my permission.
Any advice welcome.