Thanks.
A bit more info.
There is a seperate battery for starting the engine. There is no shore power, but I do have a Ring Booster pack which works as a 300W inverter and from this I can plug in a battery charger and put the output from that into the 12V circuit. This however isn't how it always was.
Originally, things appeared to work fine, though I did once leave the central heating pump on and probably completely discharged the leisure batteries. The second alternator wasn't receiving an instruction to kick in and charge the batteries (it seemed that was due to such a low voltage in the leisure batteries) but if I put some charge into the system via the aforementioned battery charger then the alternator would kick in. This no longer seems to be the case, putting the charge in isn't firing up the alternator now?
Yesterday I managed to borrow some 240V from the shore and had this in for a few hours. As mentioned, the leisure batteries were reading over 12V on the meter but I was getting no life from any 12V equipment? I'm hoping to get out and about on the boat soon to give it plenty of work, which may or may not help.
I've no idea what the box is which has 'LED' indicators, these have never lit up and looking at the various cable colours coming from it, they may correspond to some of the loose cables in the engine bay (Appear to come from that direction).
Cheers