Could she have left it with the bank in a safety deposit box or similar?
Is there someone she would have been likely to have made an executor, like a family solicitor or accountant (if there was a business or pensions involved) or closest family member who may know?
Did your grandfather have a will, it's possible they had it done by the same law firm who may have them both.
If your grandfather is not still around, could they have appointed the same executor? If you have a copy of the probate then it should say on there and may well be a law firm or a person who witnessed the will and may know where it might be.
You can get a copy of a will from the Probate office.
http://www.hmcourts-service.gov.uk/infoabout/c ivil/probate/index.htm
They also may be able to give you some further advice.
Depending on where you live it is more common for the older generation to have had a family law firm, for conveyancing, wills etc..., do you have an idea who they would have used?
Did they own their house and can you find out which solicitors were used for that as they may have been involved in making wills as it was common to do them at the same time.
Could you write to local law firms asking them to check their records and ask for a follow up confirmation whether or not they have it. A will is a deed and has to be stored in fireproof storage and there should be records of them being held which should be straightforward to check.
Hope these ideas help, if I think of anything else I'm come back to you.