Market trader operating as Ltd Company - what are invoicing requirements

I've been approached by a prospective client who is a market trader trading as a limited company. All of his sales are either cash or debit/credit card.

As a limited company I assume that he should provide a full invoice for each sale (even though this may take the form of a hand written receipt from a carbon copy sales book with the relevant company information printed on it).

Is this enough?

Thanks in advance