Whickerman is right that they are breaking the law if they haven�t put details in writing to her, but an employment contract does already exist and she is accepting its terms.
The DTI website is good for stuff like this. I think it�s written in plain English. This is an extract from the site:
�Written statement of employment particulars
All employees taken on for one month or more are entitled by law to be given, within two months of the date the employment starts, a written statement setting out the main particulars. This statement will not necessarily cover every aspect of the contract, but will constitute important evidence of the principal terms and conditions. Further information about the right to a written statement can be found in
Written statement of employment particulars - Guidance�
I don�t think they are necessarily breaking the law in any other way. And as long as she wants to carry on working for them, I guess there isn�t much you can do.
If it really bothers you and she won�t listen to you, write an anonymous note to the Tesco store manager saying you think they are in breach of employment legislation. Don�t mention your wife. She�ll lose her job when they lose the contract, but you might feel better�.