stop being a nice guy and go for the jugular. If you pay the £78 to repair or the insurers bump your policy up by a bit, you're still out of pocket so may as well give them the insurance hassle.
2010 C350 CDI Sport sold , 2015 E250 CDI AMG Line Sold
Wright the letter saying that the cost is £70 and thay have 14days for payment to reach you. If you hear nothing after the 14days then it will be reported to your insurace co to follow up. I would pay the extra in insurance just to teach them a lesson.