i had the same issue with my 2001 disco, in the end the wastegate disc had got sloppy on the arm inside the turbo and caught sometimes. i looked to get it sorted and needed the car on the road asap so bought a brand new one.
sorted my overboosting out straight away.
to remove the turbo is really easy.
3 nuts/bolts on the heat shield,
3 nuts on the turbo to manifold,
3 nuts on the turbo to exhaust,
1 on the oil feed,
2 on the oil return.
about 30mins - 1 hour to remove