Lack of trust with executor of will

[WhyBother]

My wifes grandmother has recently (sadly) passed away. In her will she left a cash sum (�20k) to her three grand children and a further cash sum (�10k) to her only great grand child, with the balance of her estate (house) to go to her son, who is also the executor of the will.

It is believed (but not yet proven) that there was sufficient cash to pay the named cash sums out to the grand/great granchildren.

Subsequently the grandchildren have been contacted to advise that they can only be payed half of their inheritance before the house is sold.
The suspicion is that the executor has taken a large protion of the available cash to ease his own financial position, which is believed to be precarious.

Can he do this?

Also, if he can and he only pays half of the inheritance to the grandchildren is there any action that they should take to ensure that they are fully paid out when the property is sold or is the will and the involvement of the probate court suuficient to ensure the instructions in the will are complied with.

Thanks in advance for any information/advice.