Mondeo won't start

Hi guys

Want to pick your brains before I spend any more money on this damn car. 

14 plate mondeo titanium x, the dreaded powershift version (lucky me!) Anyways, got it home one night fine, tried to start it the next day and nothing. Press the start button and all the electrics come on, push the brake in and go to start it up and 1 singular click, all the electrics go out and no start. 

Have had a mechanic come out and he said starter motor, so he replaced the starter motor and suprise suprise, nothing, just the singular click again. He then said something about something not connecting near the gearbox?? And off he went.

I took the battery off and charged it overnight as the battery was flat a couple dayd adtet the mechanic looked at it. Again, still singular click, no start. 

Only code we have been able to get from it (mechanics software) was to do with the fuel/dpf vaporizer, but I would imagine that this wouldn't stop the car from starting? 

Bit of a backstory. 

Have had this car since the beginning of last year, in this time it has had 

Replacement gearbox ... worked for a week then had to go back to be replaced again as it failed, was stick in park, so they accepted liability and the car was working fine again after that.

Wiring loom replaced.

New radiator.

Drive belt and tensioners/pulleys.

Dpf flush as it went into limp mode, worked fine after that.

Multiple different hoses replaced and other bits and bobs. 

I have actually spent more in repairs than on the car in the first place... gearbox alone was 4k 🙈 but I don't have much choice as its on finance. Its spent more time in the garage than what I've been able to drive it. 

Unfortunately, all the problems started after warrenty had run out.

Also only reads I have 14miles of diesel left in the car as I needed to get fuel the following morning on way to work 😅  

So  if anyone has any ideas what it could be before I end up with yet another garage/mechanic bill, I would be very grateful.