After some initial research, including looking at the cost of buying one, I decided that it would more sensible to make one.
A simple former was made out of an old shelf and two blocks of wood. The tiller was made by using seven layers of alternating wood, each 5mm thick. After an initial dry clamp to check the shape, the layers were glued together and left overnight to dry.
Once unclamped the laminated wood could be shaped. The nominal cross-section size was 35mm sq. The width was tapered down to 25mm so it was more comfortable to hold. The most difficult part was the shape the new tiller to fit correctly in the head casting.