Had the same problem with my 200tdi, i think the 300TDi is similar so could be the same thing.
There may be a build up of carbon around the injector body, at the bottom of the injector where gasses have leaked passed the copper washer.
This will make them difficult to remove, I sprayed penetrating spray around the outside of the injector so it ran in to the head and left it for a couple of hours, that did the trick.
Just be careful twisting them as I started unscrewing the injector body and didn't realise.