The simple answer is it is not unusual for a stay to take that long to be credited - although it is not supposed to!
I presume you are checking for the credit at the Hilton web site? Not waiting for a statement (which can be VERY slow in coming).
I would give HHonours a ring. I presume you are sure the stay was eligable? Ring them and fax a copy of the bill. That usually does the trick. If you still have no joy, fax it to the UK Hhonours Servcie Centre, they normally get them posted within 24 hours, they seem to be much quicker than the US.
MERRY FLYER