Hi o/s door lock packed up can only open door from outside, can any one tell me how easy is it to replace the door lock? can they be repaired?

On most cars it is simply a matter of pulling the trim off the inside (remove handle, window winder etc) and the inside of the lock can generally be accessed. Some cars have metal plates shielding them to prevent theft.
some locks are held on with a metal shim that has to be slid out, some are held on with a nut.
You say the door can only be opened from outside? That sounds more like a problem with the internal linkage from the door catch.

if it opens fro outside then the lock is fine, the problem is the rod/cable connecting the interior handle to the lock mechanism. remove the door card and reconnect it, checking for any reason why it happened in the first place.