BA are no longer operating this route and therefore will use this as a valid excuse not to take responsibility. Your best bet would be to contact MH, who will undoubtedly tell you they can't be blamed because they were not the operating carrier. Upon closer inspection someone will come up with the fact that you probably missed your connecting flight out of KUL, hence the remaining legs of your journey should have been cancelled and repriced as a new one way. BA will charge your credit card with the equivalent of a one way at full Y price. Or at least a change fee (£60 + full Y). At the very least you will loose all your AVIOS in an account audit.

Any more questions?