Hi,
The question that arises firstly, is what the demand is for. If it is for instants for payment of a debt, sufficient time should be provided, however the particulars of the time frame depends on the type of contract. In any event, if your customer were to commence any form of Civil proceedings a without prejudice notice of court action will not suffice as sufficient under the Civil Procedure Ruled, and the tenet of the pre action protocols. For any commencement of court proceedings a 7 day notice should be issued, and then draft particulars of the claim, together with any document(s) to which the claim relates. If proceedings are commenced without the prescribed notice having been provided, then you may bring this to the attention of the court concerning the question of costs. Any without prejudice letters may be presented to the court on the question of costs, however this can only occur after judgment is given, and usually it is good practice to mark any correspondence with such an intention as 'without prejudice save as to costs. If you need any more advice then you will need to provide more detail of the circumstances. Hope this helps!