This employee will be deemed to have been working for you for the week that you employed him therefore he is entitled to be paid in accordance with the contract of employment that you gave him when he commenced employment with you.
If you fail to pay, the employee would be able to bring a tribunal claim against you for unlawful deductions to pay. He would have three months within which to bring the claim.
Just because the employee did not perform any productive work for you does not mean that he is not entitled to be paid. He has performed under his contract of employment, ie turned up and participated in the training that was required of him to carry out his role and this will be deemed as ‘work’.
It does not matter if the employee had not yet been given a written contract of employment stating his terms and conditions because the concept of a ‘contract’ was already in place since the beginning of his employment with you.
His rate of pay for this week should be the rate that had been agreed between yourselves, either in the contract, or in the employee’s offer letter etc. This must be at least national minimum wage but you cannot revert to paying the employee at minimum wage if a higher rate had already been agreed.
To conclude the employment properly, the employee is entitled to any other payment that his contract entitles him to upon termination of employment, for example, accrued holiday pay, calculated at his normal hourly rate.