Thank you. She cannot claim adverse possession unless she has occupied the land (which the bridge is on) without consent or objection are not in secret for at least 10 years if the land is registered or 12 years if unregistered at the Land Registry.
She cannot claim adverse possession without lying because other people have used the bridge for many years.
You could make a Freedom of Information Request asking them how many times they have visited the bridge to inspect it and how many times they have carried work out on it in the last 10 years. Hopefully, they will be truthful!
Even assuming that she did own the bridge, if she did not own the land on the other side of the bridge, her ownership only goes to the centre of the riverbed and hence, she could only own half of it so she would have to own both sides of the river to have any chance of owning the bridge if it is in her deeds.
Also, if she does own it, then provided you and your predecessors have used it for 20 years or more for access, you can claim the right to continue to use the bridge under the Prescription Act.
If she starts to demolish the bridge, you could make an emergency application to court for an injunction to stop the work and that, if done by a solicitor attaching a certificate of urgency, could be in court within a few hours.
If she is attempting adverse possession the existing landowner (if known) would be written to but there is no harm in sending your own letter saying that you object to any application for registration of this bridge in the name of Mrs X on the basis that you have a long-standing right of access over it.
It would help if you attached a copy of the deeds of her house with the bridge adjacent so that the land Registry can see exactly where the bridge is. Provided her property is registered at the land Registry you can get the title plan and her deeds from this link.
the documents will cost you £3 each.
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