Locum Sessions

Q: The partnership has employed a part-time salaried GP for a number of years who does locum work in other practices to supplement his income. He has offered to cover the absence of one of our partners as a 'locum'. We are not sure if we can pay him a gross amount with no deductions for tax, national insurance or NHS pension contributions for these sessions. Should we instead add the sessions to his salaried employment contract and make deductions as with the rest of his pay?