A bit annoying having paid £20 to highlight or avoid future issues, and having this happen. There is supposed to be a position at these dealerships that goes by the name "dealer principle" - or there used to be, contacting him/her by recorded delivery should have worked, though as they are just throwing you back the £20 they seem to have got away with this, and so will no bother too much about being less than professional in the workshop.
I once handed in my Ford Orion Ghia 1.6 Injection for some issue under warranty to my local small Ford agent, when I got it back and got home, I checked under the bonnet and noticed that something was not right but could not work out what. After going back into my house, I suddenly thought "dipstick!" - so back out I went, and checked the dipstick, its handle was rough and blackened with age, the dipstick with its oil level sensor was black and carbon coated - ie from a very old high mileage car and not my 6 month old 4,000 mile car. I returned to that
dealership and one of the owners was in, so I demanded that the dipstick was replaced - he was livid and challenged me - I pointed out that there was a nice new Ford Orion Ghia 1.6 Injection in his showroom (luckily) and I needed its dipstick quickly - he complied, I never went back there.