Reanult Laguna 97. Weird Fault!?

Hi,
Have a weird fault here. I can sometimes arm and disarm the alarm/immobiliser. When disarmed, the car will not start, it just clicks 4 or 5 times, getting slower towards the 5th click. Sounds like battery so far. Recharged battery, measuring 12.5V. Tried again, but this time, instead of clicks, the lights on the dash flicker like xmas lights. When trying to start the car, immobiliser kicks in. The only way to start the car is jumping it with the misses car via jump leads. Now the car is running, speedo, rev counter, fuel and temp guages arent working.? Car will only start with jump leads. yet when charging the battery, its states its full.
I have done and checked.
1. Bought a new battery charger.
2. battery charged
3. Checked all Fuses
4. Checked all wired connections under the bonnet.
5. kicked the car. lol

another thing to point out is that before i charged the battery, i had to put nearly a litre of top-up to cover the plates. Day before this fault developed, i drove 250 miles.
Anyone got any ideas?
Although im getting 12.5V on the battery with multimeter, is it the amps that count, and why does the charger say its charged?