I have stupidly dropped the  speedo drive pinion into my diff I am looking at splitting the diff casing to remove it Does anybody know how difficult this is to do & what i need to look out for

This is not an easy job for DIYers or beginners. Very labour intensive. You need a decent workshop, means to jack the front of the car and to support the engine, an assorted set of socket wrenches, other various more or less special tools, and a service manual.

Splitting the box isn't too bad a job but if you have never done it before I wouldn't recommend doing it on one you want to work afterwards.

If you leave the speedo drive in there it will get mashed up by the difference and bits of it will fall into the selector mech and stop it selecting 1st 3rd and 5th.. I would just look for another gearbox and then you can strip yours at your leasure..

You need a good set of circlip pliers a 3 mm punch and a slim 32mm socket to get it apart there are no special tools needed if your just opening the casing to remove the loose bit.