Scaling from Land Registry plans is a dangerous hobby. In practice, the thickness of a line on a Land Registry plan can equate to a number of feet on the ground, depending on the scale of the plan.
Chances are that the solicitors that have written to you have good reason to suspect that the land may belong to their client. Ask them to send you a copy of the title to their client's land, including the area which they think you may have possession of.
Whilst you're doing that, ask the solicitor that acted for you when you bought the property for a copy of the "Seller's Property Information Form" that was completed by the people who sold the house to you. The first question on this form deals with boundaries - specifically if any of the boundaries have been moved in the last 20 years. If the people who sold the land to you have not answered accurately, you MAY have a case of misrepresentation against the sellers, IF you actually lose the land.