Gazelle W11?

Hi
I have a number of Westwood tractor mowers and look after them myself.
I recently purchased a Gazelle-w11 from the original owner with a rusted out cutter deck, the machine is in otherwise good condition so I decided to remove the deck and use it as a tractor to tow a trailer on my farm. All has gone well other than while it starts on the key, is charging ,lights work but there is no ignition light and if the engine is slowed to tick over the lights flicker and the engine stops. It would seem that the engine is not receiving power from the battery. I think that I have isolated the safety switches correctly as both the amber warning lights come on.
I like to keep these old machines running and would be very grateful for advice.
Best wishes,
Brian.