The grower has got back to me
'I am afraid that by allowing the shoot from the rootstock to grow you have deprived the Norfolk Royal bud of food and moisture. If you had cut off the shoot from the rootstock at the beginning of the season, the Norfolk Royal shoot would have taken all the food and moisture from the roots and would have put on more growth. However, you can still remedy the problem by cutting off the rootstock shoot as near to the stem as possible, and feeding and watering the tree. (I am assuming that, like us, you have had very little rain this summer). The Norfolk Royal shoot still has a little growing season left, and you can start again by training it up and beginning the espalier training next year'
I have posted this for anyone else in a similar situation.