Car wont start / engine wont turn over

My ford focus zetec 1.6 wouldnt start, engine wouldnt even turn over, central locking on key wouldnt unlock door, so had to unlock manually and when in the car the other doors wouldnt unlock either. Car started with a jump from another car and drove fine, next day exact same thing. Replaced car key battery didnt make a difference, replaced engine battery worked for three days no issues, came out on fourth day wouldnt unlock / turn over etc. doesn't make any noises or lights flash or anything. Mechanic came out thinking it was the alternator after checking thinks that and the starter motor are fine. I am going to try and take it to a garage tomorrow however want some ideas of problems as have been reading online people have said ford have replaced stuff costing £1400 and it turned out not to be the problem. Some people have said possibly the cam shaft sensor or loose cables. Any ideas woud be appreciated