Barmaid (who is a barrister specialising in such matters) has answered your question but, just in case any clarification is needed, I'll add just a little bit more:
The Inheritance (Provision for Family and Dependants) Act 1975 does NOT give a spouse (or anyone else) an automatic right to any part of a deceased person's estate. (So the '50%' figure quoted by B is utter nonsense). All it does is to give certain people the right to seek a court order varying the terms of a deceased person's will. The Act provides guidance as to the matters which the court can consider but it is entirely up to the court to determine whether or not the will's provisions should actually be varied. (i.e. making an application under the Act offers no guarantee that anything will change).
Further, Section 4 of the Act imposes a time limit for applications to be made: "An application for an order under section 2 of this Act shall not, except with the permission of the court, be made after the end of the period of six months from the date on which representation with respect to the estate of the deceased is first taken out . . ."
So I suggest that your response to B's demands should include the words "running jump!"
;-)