I have come across this before. Both with regard to the permission to sell the van and also the park saying that they would let it out and the lettings would cover the costs but then, they don’t and hence, it does not.
They can only charge you for selling the man or make you give them first option to purchase if you signed an agreement which said that. Although verbal agreements can be binding if the facts are not in dispute, you are unlikely to have agreed anything which they are now saying.
You may have to give them a months notice and that is not unreasonable. In order to mitigate your loss, you might want to consider selling the van and paying them their 15% commission and then suing them in the Small Claims Court to get the 15% back on the basis that you paid it under mistake or duress basically, because you had no option.
A solicitor writing to park asking them to confirm on what basis they are imposing these terms and threatening them with an application to court if they don’t allow you to sell, would be another option but it’s going to be somewhat slower.
Can I clarify anything for you?
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We can still exchange emails. Best wishes.