Spot heights are those points that the OS has measured to be exactly that height (in metres) above sea level. They are marked on the map thus: .123 always with a small dot marking the exact point. Only those spot heights where the dot is on the road should be considered. You may be asked to pass through the following spot heights:
73 55 42 88 107 66
If the shortest route passes through another spot height that is not listed and the instruction says something like "pass through these spot heights only" you should work out a route to avoid it. Always look very carefully at the location of the dot, as it may be just off of a junction, thereby not requiring the most obvious route to pass through it or avoid it.
Variations on this theme may give you the spot heights as a continuous string:
7355428810766
and may even mix spot heights with grid line numbers in the string.
You may be given consecutive pair of spot heights added together, which would be:
128 130 173
Spot heights could also be given as additions and subtractions from the previous number:
73 -18 -13 +46 +19 -41
or they may be given as additions and subtractions from an original starting point:
74 -18 -31 +15 +34 -7
Tip: Use a highlighter to show up all spot heights where the dot is on the road. This makes them much easier to see when plotting a route.