And the answer is 22.5

The third and apparently deciding measurement has taken place. And the result is [drumroll]... Twenty-two-and-a-half.

I'm keeping the workings out though as when we order - as with curtains - I will give them the size of the "hole" so they can make sure we either have enough or won't need to pave our entire garden.

It makes sense to start on the patio early, as everything has to come through the house. So from being a possible it's shot right up the list.