I ended up coming into Shoreham late with a passenger that needed dropping off. They were very helpful and although they did warn of an out of hours fee it was no where near that high (or else I wouldn't have risked it).
In the end, I made it right in time. The controller hesitated when I stated that I wanted to drop a passenger and leave again but agreed to it (I didn't power down, just taxied to the terminal to do the drop off and straight back to the runway) and I agreed to phone to organised the landing fee in the morning.
It was definitely past 20:00 by the time we were away but they didn't bother with any extra charges and in fact thanked me for expediting. On the other hand, I was well aware that I might be charged and was very grateful for the help.
It's possible that they have that high fee in order to not have to wait if it can be avoided?