Hi all and thanks for all the input!
I have both 240 and 12 volt power and only a small inverter.
I have barely been on the boat and have run a 12v TV while the engine was running the other day.
I'm pretty sure I haven't left anything switched on.
The 3 leisure batteries were tested by a friendly neighbour and they are at 12.3 each.
Yesterday I ran the engine for an hour and still had no electric power.
That's all the info I can think of.
I'll go and have another go at finding the fault.
Thanks again for all the help. It really does lessen the newbie powerless feeling!
Tony Edridge