You may be able to find out whether it actually is in her deeds by getting the Land Registry certificate for her property. Go to land register online. However, often the exact position of boundaries is not clear on the plans.
If that is the case & if the properties are quite old then the only way of checking would be to try & find the old deeds. You can't do this for your neighbour's property but you could get your solicitor to ask the Council for the original deeds for your property when you buy it. These might have more details about boundary position.