Leisure battery keeps going flat.

[teenmal]

Blown fuse and or faulty relay,I think there was a problem with some of these relays recently

[Dodgey]

Done some measurements.
With the engine off
SB 12.7v. LB 11.7v
At idle
SB 12.24v LB 11.7v
At 2000revs
SB 13.9v. LB 11.7
At 3000 revs
SB 14.05v. LB 11.7

So the LB staying at 11.7 tells me the relay is not switching over. But is this because the starter battery is not charged enough? Do I need to do this test again after a long run?

1) Your relay link is not working - I think you know that! - could be the relay, or fuse. Meter the link at the various points.
2) My battery went from 12.x (measured after glow plug warm up so missed idle voltage) to 14.22 volts, at tickover, within 20 seconds. No revving. So either your battery is duff, or your alternator is on the way out.

[Poohbear]

Sorry to be a bore but I still think the alternator is on it's way out. A good unit should easily produce about 14v at idle if that is what the battery needs. I'm no expert but have experienced a very similar situation. The control circuits in the alternator will compensate up to a point but if the generator is unable to give the necessary power then it will just about keep the SB charged but not be able to charge it quickly enough or fully in order to switch the relay to charge the LB. Typically, if it does connect the LB the resultant power draw drops the voltage and the relay opens again, sometimes giving relay chatter.

Of course it might still be a relay fault but I'm sure the Willinton kit is designed to protect the SB

Quite right. The alternator is knackered if you are only getting just over 12V at idle. The voltage sensing relay switches over at around 13.5V and you should get around 14V or just over at idle, with nothing else switched on. This means that as soon as the engine is running the relay is switched over to charge both batteries.