After I sent Iron Mountain a snottygram about the abysmal search function, I recieved the following (surprisingly helpful) reply which actually works!
Give it a try!
"Dear customer,
In order to narrow down your search. If you know the first and last name of the deceased then you can enclose the search in speech marks.
So, for example John Smith will look like this:

This will narrow down any instances of when John Smith are associated and next to each other in the probate calendar. This will significantly reduce the amount of pages that you will have to go through in order to find your details on our system.
You will still need to enter the year to search.
In the advanced search we would advise considering entering information into these fields as they sometimes confuse the search system. And can result in no information being found. You can leave these blank.
If you are looking for dates which pre-date 1996 then you are searching for the probate date, as opposed to the date of death. This date is always after the date of death and can follow onto the next year (depending on when probate was applied for and issued).
Dates after 1996 (inclusive) are searchable via the date of death.
If you have any further issues please don’t hesitate to contact us.
Kind Regards,
HMCTS Wills Helpdesk"
Give it a try!
"Dear customer,
In order to narrow down your search. If you know the first and last name of the deceased then you can enclose the search in speech marks.
So, for example John Smith will look like this:
This will narrow down any instances of when John Smith are associated and next to each other in the probate calendar. This will significantly reduce the amount of pages that you will have to go through in order to find your details on our system.
You will still need to enter the year to search.
In the advanced search we would advise considering entering information into these fields as they sometimes confuse the search system. And can result in no information being found. You can leave these blank.
If you are looking for dates which pre-date 1996 then you are searching for the probate date, as opposed to the date of death. This date is always after the date of death and can follow onto the next year (depending on when probate was applied for and issued).
Dates after 1996 (inclusive) are searchable via the date of death.
If you have any further issues please don’t hesitate to contact us.
Kind Regards,
HMCTS Wills Helpdesk"