TD5 Auto; will not select gear

Morning all, been a while since I've been on here!

Our Y plate TD5 Auto has had the odd electrical gremlin, and now would appear to have gone terminal on me.

Came back from a week away to use the disco and she started fine, left her running for. Few mins came back and the M&S lights were on and wouldn't engage gear.
Turned it off, locked up tried again etc nope.
Bought a CTek charger as I read that it might be a battery fault, thT did a recon on the battery and seemed fine. However still wouldn't move.
Left it. Few days and now it won't even kick over. Battery has been on a maint charge and is showing green on the charger.
When turning on all the lights come on, the fuel pump does it's thing etc as normal.
Try to start it. And it goes 'clunk' and all the electrics go off.

Slightly peeved now, trying. To get it sold!

Any thoughts?

Have you tryed moving into n rather be than drive

that green light shows you the status of only that first cell where it is... put a voltmeter on battery and ask somebody to crank then see the voltage...if it drops below 9V that's the M+S lights and prevous non starting reason , if it drops down below that's the reason of not cranking...if the voltage is above 9V and you only hear a loud click you need to repair the starter solenoid too, there is cheap kit for that on ebay

Or stuck starter motor

Check the earths