I've had an account with a ventinary surgery in the United Kingdom for around 10 years, who's terms of payment when opening the account were payment within 60 days.

They have changed their terms such that payment is required immediately on payment.

Their notice was in the form of an A4 sheet pinned on the wall of the vets near the reception.

My questions are:

1) Was the A4 sheet sufficient notice, or are there other legal requirements for a change in payment terms?

2) if there are additional requirements that they therefore did not follow, would a person be entitled to make payment under the old terms until the requirements are fulfilled?