Seat Leon not starting

Hi,

I parked my car in fine working order a couple of weeks ago and a week later I tried to start it and it wouldn't turn up. There was nothing happening when I tried to turn it on, no sound, no noise. The lights on the dash were still coming on fine and battery seemed to be ok as all electrics were working. I push started it next morning with the help of a few friends. A lot of black smoke came out and when I reved it, the engine cut off and I heard a noise from behind the dash ('krrrr' like a spart but not exactly) and all lights on dash came on and went off.

Since then its been parked and I had a mobile mechanic come to have a look at it a few days ago. He thought everything was ok mechanically. Battery had died so he tried to jump start but the engine tried to turn over very sluggishly and could not start. He thought that the starter wasn't getting enough power to fully turn over and start the engine so I need to show it to an electrician. I got an electrician to come out and after plugging in diagnostic tool he said there was nothing logged in so He said that the ENGINE HAS SEIZED!!!! and I need a whole new engine.

I'm very confused now, as I don't believe that the engine can seized, its only done 106k miles, I had it serviced a month ago, oil and filters and when i bought it 6 months ago it had full service history.

The only problems it had before this was a noise from fan belt on a cold start which went off after 5-10 minutes, I had the belt replaced but I was told later that it was actually the tensioner that was faulty. Also, sometimes it would take just 2 tries to start from cold but once started I could just switch off and start again and it would start instantly.

I would really appreciate any help.

Many Thanks and sorry for the long story, i just wanted to give out the whole situation.