Quick outline:-

• I left the ignition on overnight (a stupid move I know).
   
• The battery was completely dead. I tried using my wife car + jump leads to charge the battery. No luck.
   
• Started my wife's car, whilst it was running I swapped the batteries, so the fiesta battery could charge in her car, again to no avail. 
   
• Got a battery charger on trickle charge over 8 - 10 hours, battery voltage read 13.5V when I refitted it back into the fiesta.
   
• I now have power to the low voltage systems. but when I turn the key to the start position, I get nothing. No crank, No sound from the starter. (please see images)
   
• Checked all fuses & relays in the engine bay fuse box, all battery connections are secure.
   
• Have NOT attempted a bump start or used a booster pack.

From other information I've read it seems that when I let the battery go flat one of the vehicle modules might have lost connection/recognition with my key through the PATS system. I'm no positive that is the case, but signs point towards it. 

Would really appreciate any help or advice anyone on here is able to offer so I can get this thing moving again. Thank you in advance.

Key in on position before cranking:

 

Key attempting to crank:

 

Key in on position after cranking:  (note that mileage is now blank)

what does the voltage read whilst the ignition is on and dashboard lit up? you say it read 13.5v but was that only checked before you drew any power from it. I am wondering if as soon as you put a load on it (turning ignition on) the voltage immediately dropped too low for the car to attempt starting. 

ive the same problem with my ford fiesta 2009

 

whats the problem with it?

How old is the battery?