Thanks for the offer and advice!
Sorry for the delay, yes an auto electrician found it in 20 mins. It was the alternator bulb/lamp. What usually comes on when your alternator is faulty in a car, but I never knew this bulb was in the circuit as it never had worked. Lesson learnt here now. Still one alternator to repair and batteries would benefit from being changed. Also to my knowledge this system is now just 12v, I haven't found any 24v lying around anywhere yet.
Thanks for your help, learning curve here!