There's no need to pay for a form. Simply find out which Probate Registry will be dealing with the probate application (from here:
https://courttribunalfinder.service.gov.uk/search/postcode?aol=Probate ), then copy-&-paste the following:
IN THE HIGH COURT OF JUSTICE
FAMILY DIVISION
THE [district name] PROBATE REGISTRY
In the estate of [name of your MIL] deceased:
WHEREAS [name of your MIL] of [your MIL's full address] deceased, died on [date of death] made and duly executed her last will and testament dated [date of will] and in the will appointed [names of both executors] as executors.
Now I [name of executor completing the form] being one of the executors do declare that I have not intermeddled in the estate of the deceased, and will not hereafter intermeddle in the estate with intent to deceive creditors, and I do now renounce all my right and title to probate and execution of the will.
Dated this [day of month in words] day of the year two thousand and sixteen.
Signed by [signature of executor]
[Typed name of executor]
In the presence of [signature of disinterested witness, NOT a family member]
[typed name of witness]
[address of witness]
[occupation of witness]