Bay Tree with Very Different Looking New Growth

Please could you tell me what is happening with my bay leaf plant? Most of the plant has regular looking bay leaves which we use for cooking. The upper new growth leaves look quite different, longer and skinnier leaves, no smell or flavour. They seem like they are almost grafted on from a different plant. Are these actually usable bay leaves or should I prune them off? What are they?

No don't prune them off. You are seeing juvenile leaves that will eventually thicken, harden, grow deeper green in colour, and develop the characteristic bay scent and flavour. Bay is a seasonal grower, showing a burst of new green growth in spring and early summer, and then settling into a slower growing phase toward the end of the summer and into the fall and winter seasons.
If the new growth escapes the bounds of the shape you are trying to create (bush shape, tree standard shape, topiary figure, etc.) then of course, prune the new growth as needed in order to preserve the desired shape. Otherwise, leave the new growth alone.