Hello and thanks for using Just Answer.
I am Al and am happy to assist you with your enquiry.
You should serve notice on your neighbour by writing to her pointing out your concern regarding the damp/disrepair and advising that you are concerned regarding potential future damage to your property and inviting her to attend to the matter as soon as possible and fully reserving your rights in respect of any possible damage caused to the property. You may wish to keep a copy of any such notice you send.
If any damage does occur to your property you would, particularly having sent the above, have a clear case against your neighbour both in respect of negligence and in respect of trepass occasioning damage to your property. You would have a claim in respect of both the cost of repairs to your property and if it were necessary to prevent further damp/water damage in the future the right to consider an application for an injunction against your neighbour in respect of the flow of water onto your property.
If her failure to repair her house is resulting in damage to your house you have a claim against her. The above notice is not a pre requirement to make a claim but it is worth considering because if damage does occur at a later date and you have a copy of the notice you send to her available it would not be possible for her to claim that she had no idea that damage was occuring to your property. This way you have her for negligence as well as trespass occasioning damage and the claim would be very hard for her to defend against.
If you need to claim monies from her in the future the easiest way to do so having obtained quotes for repair and asked her to pay for the same is to use www.moneyclaim.gov.uk
If you need to seek an injunction against her to prevent further damage occuring you will need to issue County Court proceedings-form N16A.
Hopefully, such action won't be required.
I hope this assists you and sets out the legal position.