Applemaid,
Dereks advice about you not risking any sort of comeback via a tribunal etc, is I am afraid quite wrong, there have been many instances where employers have lost just these sort of cases, so I would advise you to take "Real Legal" advice on this.
Taking the above action, will of course, cost real money.
Dealing with employee's who threaten leagal action of any sort is fraught with serious danger if you have not followed corect procedures.
You may feel, that because you have E/L Insurance that you covered, however, in the last few months, I have heard that some insurance companies have refused to cover some employment claims because the employer did not follow the correct procedures, putting two reasonable size firms out of business.
Regarding the un-authorised overtime, surely this is covered within your employment contract/handbook etc. So no problem there!
Regards,
Rob