Christian McArdle wrote

We were planning to get an ultra large scale plan from the OS and photocopy
it at a few % enlargement and write "Not to scale" on it.

LOL! That's good one - I'm going to remember that, cheers. But, from
experience, the OS digital plans are quite inaccurate anyway. Many times I've
measured an existing building, drawn it in AutoCAD and then found it won't fit
on the OS site plan!

Peter