Accordions are very different from concertinas, and whilst they both contain free reeds they are of different types. I would be cautious about letting an accordion shop carry out repairs unless they already have a reputation for working on concertinas as well. You'll do better to find a concertina specialist - concertina.net will point you in the right direction.

Get hold of Dave Elliott's "Handbook of Concertina Maintenance". This will show you how to carry out many repairs. Many problems can be solved fairly easily, and even if you decide not to attempt them yourself it is useful to understand what is involved and when it might be advisable to bring in an expert.