Senecio:

 

I've just received a reply from the motorhome company. They explained the factors at play in their pricing model, which was all accurately covered in this thread. However they also acknowledged that it was a steep increase and with no expectation of the price declining anytime soon they have agreed to a full refund of my deposit which I have gratefully accepted. I understand that this was not standard policy and more than I was entitled to so am extremely grateful for a good outcome in this situation.

 

That seems a very fair outcome for you and them.