I have just heard from BMW customer service that in order to fully diagnose the wastegate issue, before they are able to submit a 'good will claim', they need the units to be removed from the car and inspected. The cost of such diagnosis is £1500 + vat.
They have apologised that they cannot help me any further (yea thanks!), but without a diagnosis they cannot submit the claim.
I stuck my hand down the side of the engine over the weekend, and managed to give the rear wastegate actuator rod a wobble, and its wobbles a lot. the rod and locking nut are tight, but the wastegate are have a lot of movement.