Steve Naïve writes
Just had another thought - combine the 187 and the 31 to make a 328.

I would have thought that using the general principle of Busmaths; if
you combine a 187 & a 31, you should get a 218 and not a 328? Is this
an example of the Brondesbury Bus-Bunching Co-efficient?

I'll get me coat...

--
Dave