Hi John. Very unusual to be the injector itself. Usually the wiring loom / ecu. The oil leaks from the cylinder head, down the loom (inside the core of the wires) via capilery effect & dumps oil into the ecu, causing loads of misfire codes. You can try taking the ecu multiplug off & cleaning all the connections. I have had to renew an ecu to cure. There is a modified loom, but the oil still gets up that. Defenders usually have the seatbox (where the ecu is) floating in oil!
If you really want to remove the injectors, remove cam cover, inj harness. Loosen all rocker adjusters to injector pumps. Remove rocker bar. Remove clamp. make a note of injector letter stamped on each injector. There is a special injector puller tool LRT-12-154/1 to remove the injectors. This prevents you bending/breaking the injector tips. Once the injector is removed, bring that pistion to the top & suck fuel out of bore with a syringe. Renew injector seals. When refitting, with the rocker on max lift, adjust the adjuster right down until it bottoms out & then unscrew 1 turn & lock up.
You will need to re-code in the old/ new injector numbers to the cylinders with T4 or similar, else the engine won't rev.