I bought Mike's bike. I had it checked by a fellow Cycle Club member. It did indeed have a flat spot. I rang 50cycles to ensure that the rim was centrally placed in relation to the axle.
The chainstays are not uniform and the Nuvinci gubbins is added on the outside of the rear sprocket. Also the wheel was not absolutely centred and one brake block was completely worn whilst the other side had no wear. I was given short shrift, the mechanic saying he wasn't going to tell me how to build a wheel over the phone.
Having extracted the information that the chainstays are equidistant to the centreline of the wheel, I took it to my local bike shop and he has rebuilt it with a Mavic rim. All is now fine and dandy. £75.
I now need to bleed the brakes to get them working in a uniform way.
Maybe the 50cycles mechanic was under pressure but he wasn't listening to what I was saying, just questioning the ability of me and my wheel builder, who, incidentally, mechanics for Great Britain triathlon team.