This is covered by the Direct Debit Guarantee into which all banks and building societies which run the direct debit scheme subscribe. The guarantee says:
The direct debit guarantee applies to all banks and building societies taking part in the direct debit scheme. It says that:
if there is a change in the amount to be paid or the payment date, the person receiving the payment (the originator) must notify the customer in advance.
if the originator or the bank/building society makes an error, the customer is guaranteed a full and immediate refund of the amount paid.
customers can cancel a direct debit at any time by writing to their bank or building society.
Assuming it is a direct debit not a payment taken via your debit card, they must either notify you of a change 30 days in advance or stick to the agreed payment date. If tis is not the case, you can claim back the payment from your bank, speak to your bank about it and they will probably contact VP or their bank coz they will not to want to pay it back to you every time the date changes.
I cannot understand why they should want to keep changing the date unless payment is 4 weekly rather than calandar monthly but this would be very strange.
Your statement should also advise when payment s to be taken.
