Thanks everyone for the info, Just had some info semt by Kinglsey for comparison:
Robinson's engine (80in.) at South Crofty built 1854 was 335hp.
Taylor's engine at East Pool & Agar (90in.), built 1892 is rated at 425hp.
I have tried using the formula but my answers are out by a magnatude, think I have unit problems. Schoolboy error stuff!I'll keep trying changing the units until somthing looks semi sensible.
Looking at the No 1 engine at Crofton the HP required to lift 12 tons/min to 40 feet would be very roughly 32 HP.
A rough calculation of the steam cylinder HP at 12 strokes/min seems to come out at 72.5 HP (at an MEP of 6 PSI @ 26 HG Vacuum).
An 80 inch piston is 3.26 times the area and would work out at 236 HP for the same steam pressure, I would guess that South Crofty had a MEP of 9 PSI.
The difference between the apparent HP required and the HP available from the engine is all to do with friction and the power required to accelerate the mass of the engine/water column from rest each stoke.
Edit: No allowance needed at Crofton for increased discharge head.