If you fancy a trip to Shebbear, that is an hour and a bit from Junction 27 on the M5, I reckon our resident gunsmith will do it for you. He has jigged up to remove AT from the 400 series.
But be aware that AA may have been more cunning than just plugging the transfer port adjuster. It is likely that the reg pressure has been set high, and compensated by winding up the hammer spring pre-load, all this done with the TP wide open, so there is no further upwards adjustment available without a strip down and a mod to the reg. And the reg will be done up so tight you need something like 600 Nm of torque to undo it.