I've just fitted a split charge system using a Durite voltage sensing relay (a kit from 'Simply Split Charge') to my TD5 110.
The relay is calibrated to connect the batteries at 13.3V and disconnect at 12.65V and works both ways so a charge on either battery activates the relay and connects the batteries.
The leisure battery is new and fully charged and measures at 13.06V, my starter battery measures 12.62V - neither enough to cut-in the relay.
When I start the car the relay cuts in and connects the batteries so they are both charging (as expected), however, when I turn the engine off, because both batteries are above the cut-out voltage of 12.65V, the relay stays on and the batteries connected.
I'm assuming that in this state, the leisure battery is actually charging the starter battery - not a bad thing - but as I start to use accessories I guess they will draw on both batteries until both reach 12.65V when the relay will cut out and the leisure battery alone will be used.
Is this correct and is this the intended operation of a split charge system? I was assuming that they batteries would always be isolated with the engine off.

As is, your batteries should balance out at an equal voltage with no load, then as you state once the voltages drop enough the relay should click over and disconnect the pair. Personally I prefer a complete disconnect when the alternator is not running, that way if I do have a problem I should be able to jumpstart myself from the second battery. I use a simple, cheap caravan shop split charge controller that has a small adjusting screw so I can determine the voltage at which it trips, and have set it quite high so that the change occurs with the alternator being running or not.

I does the relay allow at least one battery to be connected, if it don't you will screw the alternator.
Why not fit some diodes in the lines to split the batteries when engine is not running, if not you will kill both slowly at the same time.

That's one of the problems with the split charge relays over a proper duel battery management system. I've seen people wire in an extra solenoid triggered from a switch ignition live, so to trigger the voltage sensing relay the ignition has to be on and voltage over 13.3v.

To be honest I wasnt expecting it to work like this, and I think with hindsight I would prefer a heavy duty standard relay with a trigger from the alternator/ignition instead of the VSR, that way I would know the batteries are isolated with the ignition off.
My main concern though is that my fully-charged starter battery is only reading 12.62V (which is below the relay cut off) but the batteries are still connected because the new leisure battery is 13.06V. So with the engine off won't both batteries be discharged until the highest voltage one (the leisure battery) is below the 12.65V cut-off? But by that time I'm concerned that the starter battery will be well below 12.62V - or will they balance out before this happens? Or will the accessories draw from the higher voltage leisure battery before depleting the starter battery?
Mcclegend - I like the solenoid idea, best of both worlds and peace of mind - just thinking a cheaper way to acheive the same result would be to use a standard low-power relay to switch the VSR's earth lead when the ignition is on, therefore when the ignition is not on, the VSR would not be earthed and would be disconnected - would that work?

Did you measure voltage at the bat clamps or on the bat poles, not so say to loose a little due to corrosion on e lead.

All new clamps and leads thoughout the system - but by concern isnt what the voltage is readying its what voltage level the start battery will reach before the VSR cuts out. I'm hoping that he leisure battery will top it up or the accessories will draw from the higher voltage leisure battery first and not deplete the starter battery before they disconnect.

Yeah relay on the earth side of the vsr should work.

With the relay active both batteries can be classed as one battery and any accessory would just run off both until the voltage drop across the batteries goes below the 12.6v and then the starter battery would isolated.

Sounds like my ideal solution then, thanks Mcclegend. Thanks all for your help.

It's worth carrying a foot long (or so) jump lead so you can manually connect the positive terminals on the battery together to jump the starter battery from the second battery if needed.

That was my next plan - I'm going to permanently connect the batteries via a manual isolator switch for emergency jump starts.