Unless a will contains a specific clause stating that it is written 'in anticipation of marriage', it is automatically revoked by marriage.
If your father's will did not contain such a clause, his will was automatically revoked by his marriage. (i.e. the document his wife wrote on was just a meaningless piece of paper, since he longer had a valid will). He died intestate and the rules relating to intestacy will apply.
If your father's will contained the relevant clause relating to his forthcoming marriage, only the terms of the will which were written down when he signed the document (and his signature was properly witnessed) are valid. Anything which was added afterwards (whether by your father, other than in a properly witnessed codicil, or by anyone else, whether before or after his death) has no validity.
If you believe that your father's will was still valid (because it specifically stated that it was written 'in anticipation of marriage') and you're concerned that its provisions may not be properly carried out by the executors, you should contact the Probate Service:
http://www.hmcourts-service.gov.uk/infoabout/c ivil/probate/registries.htm
Chris