John - well understand your problem. Probably due to the hitch being slightly compressed on way or the other when you were trying to unhitch. The usual problem is after reversing the van onto the pitch.
When you first couple up, look at the hitch area of the outfit and try to memorise how the convoluted rubber gaiter looks.
Before trying to unhitch, look at this again, and if it is either more stretched out or (more likely) more compressed than yo remember, put on the handbrake of the caravan and then more the car very gently to either stretch or compress the gaiter to the sort of position you remember. Hopefully, you will thien find the coupling will lift straight off (so long as you remember to lift the hitch handle - not the big red stabiliser handle which should have been lifteD upright before.
In extremeis, wind down the jockey wheel so there is a little upward load on the jockey wheel, get someone to hold the hitch handle up and then move the car very gently back and forth. Have the van handbrake on and be careful. In the worst case if the hitch is compressed, when it releases it may extend forwards and damage the back of the car.
Best try this out a few times on your home ground to get used to the positions. Being on a slope further complicates matters,
Hope this is of some help.