ianniann, geegas, transam - thanks for your sensible comments to my question. My red dial does a full revolution in 40 seconds. 1092/40=27.3. A little high maybe and I have no intention of undoing anything.
Many thanks
Huw
Boilerman2 - my attidude was a direct response to 2 very unhelpful posts. (45yearsagasman and twgas). You 3 might want to consider your own 'attidudes' first.
I make it 3600 x 1040
40 equals 93600
div 3412 equals 27.43 div 1.11 equals 24.71 kw fine by me