Hello again!
It's still beeping - but we have more information about the situation!
So we've bought a generator to charge all the batteries and charged them all at 13.5V for about 3hours. The 'Phoenix multi-control' went from 'bulk' to 'absorption' and then when it hit 'float' we finally turned it off. So that may have saved the leisure batteries!
However, even when the batteries were nearly charged, we tried again to turn the engine on (Beta Marine) and the small beeper behind the ignition started to beep again, more or less constantly, sometimes intermittently. We moved the boat and also tried to just rev it on neutral, but it kept going (thanks anyway rustydiver!! ). The alternator works, the belts are all tight and when the engine is running the voltmeter across the red to the black terminal of the alternator says 12.8V. We checked each battery separately and they were all fine. It seemed that all the connections were fine - but then again, that's just looking with my eyes. My dad and I spent all day Saturday trying to work this out and are now happy about the batteries, but puzzled still as to what might be wrong...
Tony Brooks: which silver knob/ which button/which arm? I would like to try, but am a bit confused :s
Any further ideas as to what on earth might be going on?!
Many thanks!