In order to obtain probate, the Executors must first obtain details of the deceased person's assets and liabilities. They are then required to swear an oath (or to 'affirm') that they have provided the correct information to the Court of Probate. Only then can a Grant of Representation be issued.
Once the Grant is issued it becomes a public document, so that anyone can see the value of the estate and to whom it should be distributed.
If the executors were, for example, to deliberately omit to include the assets of a bank account in their statement to the Court of Probate, they'd be committing a serious criminal offence (quite probably leading to a substantial prison sentence) by swearing an oath (or affirming) that they'd provided full and accurate information. Further, if they then used that false information to retain monies that should have been distributed to the other beneficiaries, they'd be guilty of 'fraud by false representation', which carries a maximum 10-year prison sentence.
Additionally, the beneficiaries who'd not received their full share of the estate could also use civil law to sue the Executors for compensation.
In the circumstances you describe, it's essential that the beneficiaries who aren't Executors get hold of a copy of the Grant of Representation, in order that they can
(a) read the exact contents of the Will ; and
(b) see what the value of the estate was declared to be by the Executors:
http://www.justice.gov.uk/courts/probate/copies-of-grants-wills