All the other people mentioned have good reputations as tuners and repairers and also know about all aspects of concertinas, although as Steve points out if all that is required is tuning then expertise in other areas is of little relevance.

I have had instruments fettled by one of the top makers and also by reputable tuner/repairers. They all did excellent jobs. In the case of the maker, the instrument was away for several months. With others it took only a few days. If my Dipper needed work I would prefer to take it back to Colin, simply because he made it. However with my other concertinas I would go to any one of several repairers who are closer to me and who would probably do the work a lot quicker.

I'm not suggesting you shouldn't consider going to a maker, but I see no reason to prefer them over other recognised repairers. As a professional player, GSS may find he gets a more rapid service than other customers will. Do get an estimate of time as well as cost.