The simple answer is no to cleaning them yourself, and if your garage want £50 each to check them they're robbing you unless they have the right equipment to do it. You need the correct testing equipment to test diesel fuel injectors, and so the best people to do it are a diesel fuel injection specialist.

You can clean and re-shim diesel injectors. I was taught how to do it when I worked for a diesel engine manufacturer - it's not particularly complicated as long as you work clean, you just need a pop-tester for conventional injectors, some shims, and a set of nozzle cleaning wires and the knowledge of what the spray pattern should look like and the sound of the injector on the tester.

Advise contacting Diesel Bob to ask his advice - good bloke, tells it like it is, knows his stuff,one man band workshop with all the right kit.

www.dieselbob.co.uk/injectors.aspx

Or if you want or need reconditioned injectors depending on the make fitted:-

Next door to Diesel Bob.

www.ribblevalleydiesels.co.uk/product-category/c48.../