firstly, get yourself a voltmeter. It doesn't matter if you're not technical, it's just reading numbers off a dial. You can get a "good enough" one from Maplins for about £6.
http://www.maplin.co.uk/Module.aspx?Tab ... 79&doy=6m8
set it to 20vdc range
engine off
put the red lead on the battey +ve, black lead on battery -ve.
repeat for other battery
if either is below 12v it is not getting charged. If they are above 13v they are well charged.
start engine
repeat above test
BOTH batteries should read (almost) the same, about 14v. If so everything is working fine and your battery is at fault.
If only the starter battery is showing 14v and the LB hasn't changed the split charge circuit is at fault.
If neither have changed the alternator power isn't getting through.
If you have now identified the fault as a split charge one trace the wires from each battery +ve. There
should (but not always is) be a fuse near each battery. Check & replace the fuse. Before starting the engine recharge your flat battery with a mains charger. *
If you have identified the fault as being from the alternator you need to check that it is delivering 14v. This can be done with the meter across the big terminal and a shiney bit of metal (engine should do) with the engine running. If you don't feel confident a garage can do this easilly for you, and they may get the job of fitting a new alternator or tightening the "fan belt" for you.
* the reason you should bench charge first is that a flat battery will denad too much current on start up for any fuses you may have replaced. This is a short fall of using a split charge relay. Providing you don't discharge too far its not a problem.
