207 GTI not turning

[PieDinner]

Hi all, (my first post so do not flame me). I have a 207 GTI which I've had for 3 years. She's great but I knew I would have to change the chain as I've been getting faults and losing the turbo, however clearing restores power.
I left the car standing for a while because of this and the battery went flat..I connected the charger to the battery whilst on the car (don't shout) and when I tried to start the car it does the following:
Ignition comes on but then I get a raft of error messgaes about headlights, emissions etc. When I try and start the car there is no attempt to turn the engine at all. It's like an immobiliser fault. Also the battery light is flashing?
I found a similar thread on here for BSI reset? I tried this but nothing changed.
I'm loathed to spend big money as the cars 2008, but only done 42k and it's a nice car .
Does anyone have any BSI advice for the 207 GTI or any relay thoughts?

I've put a brand new battery on now and checked all fuses in box under bonnet and glovebox.

Thank you all, Mike

[GiveMeABreak]

It's possible by charging the battery on the vehicle that you've possibly fried some of the sensitive ECUs - especially if you've reversed the polarity.

If the immobiliser was at fault, it would still turn over the starter - but would not allow the engine to start. The fact that it's not even doing that is concerning.

I would start with the obvious by getting your multimeter out and testing all the fuse boxes (fuses in the passenger compartment attached to the BSI) as well as the engine compartment fuse boxes. Make sure all of them are working and not just visually inspecting them as you won't pick up hairline faults.

If you have checked all the wiring and earthing points, then you may need an auto electrician to pinpoint the problem - as internal ECU damage is hard to detect without knowing what you are looking for. There are a significant number of ECUs, but primarily the BSI and or the Engine ECU are the main 2.

[xantia_v6]

I presume that this is a THP engine?

Don't ignore the possibility that the starter has failed and the other symptoms that you have are not directly related.

With the new battery are you still getting a Christmas tree of warning lights and messages?

It will probablyy be necessary to have the fault codes read and cleared before you can make much more progress.