200 Tdi battery discharging?

780 amp battery, 18 months old, no previous problems. sunday morning, nice and warm, flat battery.jump leads, no probs, car used all day. next day, flat battery.
battery charged with charger overnight.next morning, OK, after work OK, next morning, flat battery.Then discovered when reconnecting poss. lead onto battery, a small
arc, suggesting a flow of current, so, all fuses out but still a small arc. after another re-charge, all OK and still OK when left with battery poss. lead disconected untill needed.
so my thoughts are that the battery is discharging through something not on the fused side of things, but what can it be?
what checks can I make to isolate and solve this problem?

now then, had same proplem once ,turned out to be earthing strap clean and refix sorted mine could give it a try! good luck.

fit an inline kill switch, doesn't find problem granted but will solve it and help with security

The only things that should be connected before fuses and ignition are the alternator and the starter solenoid.
Maybe the diode pack is faulty on the alternator?
The only way to solve this is to be thourough and start working from the battery.
Disconnect things one at a time and measure any current flow. If you dont have a multi meter (be careful because there may be high currents enough to frag the the meter) use a bulb with fly leads between the pos terminal and the pos lead. If it lights you've got current flow.
I'd start by taking the supply lead to the fuse panel off first. This will tell you if the problem is in the heavy cabling of the starter and alternator circuits.
If all is well then I'd start looking at the ignition switch circuits to the fuel solenoid and glow plugs etc.
A simple check is whith the ignition off see if you've got voltage at the various supply terminals. If so then the ignition switch is suspect.
Other than that it can be like looking for a needle in a haystack.
Good luck.