Matt Dunbar (Trinity Maths) proposes using a trapezium picture:
in a similar vein (on the NCETM site - not sure whose idea it was)
when there is any ambiguity you follow a left to right procedure
using all of the digits 1 to 9, once only, can students see how the numbers at the ends are created?
these puzzles do follow the bidmas rules
you use 1 to 9 once only to complete the puzzles