Disco Electrical gremlins...help !

Hi all, had my 200tdi for over 6 months, central locking/alarm has allways been faultless as is starting....however now have a problem

she unlocked and started fine last weekend....went out today, first she wouldn't unlock at all...no central locking working or alarm turning off, got in with the key and eventually central locking is working and the alarm appears to be turning on/off with the fob however she won't start.
Turn to position 2 the lights come on ok, but on turning to start she doesn't...I get a clicking from a relay in the passenger footwell, then the power goes off and the alarm triggers ????!!!! I know the power goes off as the clock goes off and my sony stereo beeps as it does when you turn the ignition off.

Nothing has changed with my car apart from a trip to Slindon 3 weeks ago where I did get water inside...my carpets are still wet and the car has been sitting so a possibility of condensation somewhere ??

I have tried starting whilst battery connected to my other car incase battery low, no difference, and also tried disconnecting battery and re-connecting, no difference

I don't know where to start so all opinions welcome

When you mention

"Turn to position 2 the lights come on ok, but on turning to start she doesn't..."

do you mean that it doesn't turn over or turns and does not start?

If no turn over I would start by checking battery voltage with a meter and see what drop you get when turning key

When you turn key dash lights come on and go really dim but no engine turning, I would be looking at engine earths and the starter motor

I would also suspect immobiliser, do a search on here and find out how to bypass it its a wire from battery to somewhere can't remember where

Those 2 would be my personal culprits