Thank you all for your input.
I carry out the calculations one way and my work partner carries them out the other way. The results more often than not are very similar so I shall work on whichever is the greatest for safety sake.
Both mats would be solid and we occasionally have to use a one...
If a smaller mat is laid on top of a larger one, do you calculate the GBP to each individual mat to get an overall GBP. ie GBP of 10t/sqm onto a 2m x 1m mat, then onto a 5m x 3m mat. Or is the calculation straight onto the bottom mat.