OK, many thanks for the info and advice, following on your comments
regarding building books, I remembered that I had a set of "Caxton Modern
Practical Building" which I bought many years ago in a market stall for £8,
they are the 1950 3rd edition.
Although they are no longer "Modern" they still have a lot of practical
information which I've found useful in the past. So I looked up Dumpy level
and sure enough I found a chapter on surveying & levelling and how to use
the dumpy level and more importantly a section on " The causes of error in
levelling".

Thanks again

Don.