I'm new to welfare rights for a housing association dealing with, among other things, overpayments.

Many of the overpayment notification letters I'm getting where the OP is being deducted from future benefit refer to future benefit periods that have an end date ( e.g., 19th June 2004 - 27th Dec 2004). But there are no end dates.

Does this in any way invalidate the overpayment notification or even the decision? If so should tenants appeal on the grounds the letter is technically incorrect?

Any advice welcome.

The balance of caselaw recently seems to be to the effect that minor defects do not invalidate the notice- so I think the answer to your question is no

Obviously you need to know why the overpayment occured so you can accuately advise tenants of their rights, and ensure the local office is raising correct overpayments, to do this you need to know the reason and the period covered. The Housing Association will be the loser if it is allowed to drag on.