Land sold in Poland... any problems getting money sent to U.K.?

You will be asked. And if they are not happy with the explanation, the cash will be confiscated.

For money laundering rules, you need to be able to account for any cash over 10,000EUR going through an airport or port or being paid into a bank.

If there is no father and also no other will, siblings are entitled to equal shares of any property and debt. This is unless there have already been provisions made during the life of your mother where property has been since over via a notary, in which case you will be entitled to a 1/4 of your legal entitlement unless you have received something from the estate at the time of the transfer. However assuming there has been no such transfer and I presum the family in Poland had complex PoA to sell the land, you will be entitled to an equal share. You will need to pay tax on the sale proceeds unless the land was passed onto your family free of charge with the provision that they must be you a certain amount on the sale. In this case, it will be tax free, but your family will need to pay taxes if the property was sold within 5 tax years of the signing over. Plenty of permutations here but I would localise first if there was any previous transfer or whether the family was using a PoA document to operate over your mothers land ie. whether the land sold was sold by your mother via PoA or had already been transferred to the family and then sold, as it has tax implications.

Father passed away 2 years ago. Mother signed PofA in favour of Relative maybe 10 years ago. Father had a letter telling him he had to attend a Polish Court about 3 years ago .. but was to ill. This Relative has contacted me out of the blue.

Do you remember what the hearing was about?