Suart, Dave, Simon,
Many thanks gentlemen for your collective response. The aeroplane is a CenturyUK Cessna 182, which, when I bought it, some time ago used a smaller battery with 7 cells. This model is now quoted as using a 9.6 volt 8 cell battery. The
[u]liklihood is that the speed controller is the problem, being the original one,
but the symptoms are exactly the same as when I first tried it with the original battery, fully charged, which would seem to make that equally unlikley as well

I dont have a wattmeter at the moment but the need for one is now evident. When the problem is solved I`ll put it online for the benefit of anyone buying a new battery for this model.
Gerard Jordan 4001