Is a term that means if the case goes to court, the letter is not binding in court.It is used , usually, when parties are negotiating a settlement so an offer could be changed if the other side won't settle amicably.
As john bunyan points out, they can't be held to the contents of the letter when the 'without prejudice' clause is used. That is they are not legally responsible. If you renewed your membership for a year on the expectation that it would just be £2 a month dearer and then they doubled the fee a month later once you were tied in, you would have no comeback. You need to find a copy of the original contract that you signed. It may be that they are just leaving the door open for themselves to further increase the monthly fee later in the year and don't want to be held to the £2 increase for any set period. Questions when you find the contract: Do they have the right to vary the monthly fees? How much notice do they need to give you?