Thanks all for the input.
I called BA and they confirmed that it's possible on my fare type and there's availability in the right fare bucket. Moving to LCY-AMS is cost neutral so they can make it open jaw with the £100 change fee + £3.74 in additional taxes.
Work are agreeable to then fly me AMS-MUC (KL looking most likely) instead of back to London so pending some confirmation, that's the path I'll take, will only cost work about £50 more than if they were booking the LON-AMS-LON flight from scratch (as the cheapest flights don't get me there/back at the right times. It helps to have a fairly flexible employer in this case.
Only downside for this option is I'll earn half the QF status credits I would have originally (they seem to have a lower category than BA do for partner earn), but based on travel already booked/flown I'm pretty sure I'll comfortably retain my status this coming year so it's far from the end of the world!