This is relatively easy to deal with provided the executor does not dispute that this money is owed. If they do, then it's going to have to go to court
Basically, you pursue the estate through the executors and send them a copy of the cheque and the returned notes from the bank and asking them to pay.
If they do not pay, you issue Small Claims Court proceedings against the executors on behalf of the estate.
The court will then decide whether you are entitled to this money or not although in most cases, the executors would pay up without going to court particularly if they can see that this is likely to have been the deceased ladies cheque and provided they can see the shower in situ.
Unfortunately, they cannot pay the debts until probate is granted but nonetheless, keep pressing them for this. Even if you issue legal proceedings now, they could be between six months and 12 months before they got court and that is plenty of time for them to get probate.
Put everything in writing and do not rely on telephone calls to the executor.
You can issue Small Claims Court proceedings by following this link www.moneyclaim.gov.uk
Can I clarify anything for you?