Did your grandfather have a will that advised of the property he owned?
In order to object to an application for adverse possession, you must provide a written statement to Land Registry providing the grounds for objection. If you follow the link below, there is a form that can be used to object which should have been included with the letter your mother received.
You can request that the application be dealt with under paragraph 5 of Schedule 6 to the Land Registration Act 2002 in which case the Land Registry will reject the application unless the applicant can prove one of three grounds which are in practice extremely difficult to satisfy:
5(1)If an application under paragraph 1 is required to be dealt with under this paragraph, the applicant is only entitled to be registered as the new proprietor of the estate if any of the following conditions is met.
(2)The first condition is that—
(a)it would be unconscionable because of an equity by estoppel for the registered proprietor to seek to dispossess the applicant, and
(b)the circumstances are such that the applicant ought to be registered as the proprietor.
(3)The second condition is that the applicant is for some other reason entitled to be registered as the proprietor of the estate.
(4)The third condition is that—
(a)the land to which the application relates is adjacent to land belonging to the applicant,
(b)the exact line of the boundary between the two has not been determined under rules under section 60,
(c)for at least ten years of the period of adverse possession ending on the date of the application, the applicant (or any predecessor in title) reasonably believed that the land to which the application relates belonged to him, and
(d)the estate to which the application relates was registered more than one year prior to the date of the application.
I hope this helps?