Pip, here are some considerations to muse over.
I have an ex-Salvationist Lachenal, and my experience is somewhat limited. The best concertina I ever handled was a baritone Lachenal, and he wouldn't sell it to me! It showed me how different concertinas can be. I would have paid six times the cost of mine for that one! About six weeks wages!
Quality concertinas are holding their price better than lots of other things. They are not disposable items.

Wheatstones and Lachenals are like old Bently cars. They have outlived their first (often second, too) owners, and are still going.

Concertinas sound fine, until someone plays an accordian or melodeon, and then they sound very thin.

If you buy from someone who isn't a specialist who has a selection there, do take an experienced friend with you.