Hello and thanks for using Just Answer.
I am Al and am happy to assist you with your enquiry.
First of all, you/your Solicitor should check your Deeds to see if there is any specific mention of a right to use this access. If there is, then obvioulsy you are entitled to ask your neighbour to remove the fence and re-instate a gate to allow you access.
If there is no written right to use the access in your Deeds, not all is lost. Under law, a party acquires a "prescriptive easement" if he has been exercising a right for 20 years or more. It doesn't have to be every day for 20 years- a couple of times a year is sufficient. A prescriptive easement means you will have acquired a legal right to continue to use the access into your back garden. Legally, such a right is automatice after 20 years has elapsed, but it is always best to document it formally, by way of making an application to the Land Registry, setting out the circumstances/your usage of the passageway. If successfull, the Land Registry will then put a note on your Deeds and your neighbour's Deeds to show that such a right exists.
On a side point-As your neighbour is selling, he is under a duty to notify his Buyer of any disputes relating to the house. If he does not, then his Buyer could sue him if he only gets to find out about this issue once he has moved in. I would therefore suggest that you remind your neighbour of this fact, and it wouldn't do you any harm in writing to his Estate Agent also, as they would then be under a duty to notify the Buyer. In turn, the Buyer will not want to proceed until any disagreement has been resolved and this should make your neighbour see sense and rectify matters sooner rather than later.
I hope this assists and sets out the legal position.