it is possible for the pistons, when running on worn thin pads, to come out a long way and get dirt on them, which prevents then retracting freely when you fit thick new pads.
A pro fitter or a keen amateur would have given them a good clean
Brake cleaner spray is expensive, but an old toothbrush with a bit of brake fluid is cheap
You might need new rubber boots, or if unlucky, new rubber seals which is a lot more work
I don't know Transits but this is a common prob. I would have thought a reputable high-street Brake and Tyre fitter would know how to do it but staff standards very.