“‘Average weekly earnings’, in relation to any worker, means 1/52 of his gross earnings:

“Provided that where during the period of employment the worker is unable to work because of sickness or injury, or is absent from work while on protected voluntary service or training (within the meaning of the Volunteers Employment Protection Act 1973), the divisor of 52 referred to in this definition shall be reduced by the number of complete weeks during which the worker was so unable to work or was absent from work; and in that case the term ‘gross earnings’, in relation to that worker, shall not include any sick pay or pay in respect of any such service or training, received by him in respect of any such week of inability to work or service or training:”.