As I’m hopeful I’ve cracked this problem now I thought it right that I share the measures I have taken so they may be of use to someone else in future. As I’ve taken several measures at the same time there’s no way to know definitively what’s been most useful but the order here is based on my gut feeling:
1. Position myself better on the bike. At the Bike Fit the fitter observed that my knuckles were white when riding. When I thought about this some more I realised I did hunch my shoulders and put a lot of weight through my hands. I have since concentrated on keeping my shoulders loose and distributing my weight better.
2. Perform neck stretches. I bought the book Treat Your Own Neck and used this as a basis for introducing some neck stretches. In particular I try to do these before I ride and then whilst I am stopped during a ride.
3. Use a shorter, angled stem. I replaced my stock stem with a 60mm Richey stem with a 35deg angle. I’ve found the altered position fine but my feeling is that this has been less useful than the other actions.