ESP & Flashing ECL

So I was driving on the motorway the other
day, 3 days after buying my Audi A3 off a
private seller..

Driving down the motorway we decide to
see what we can get out of it, got to
90mph & the car started slowing down.  A
few seconds later the ESP light came on
the bam in a second The ECL starter
flashing & I didn't know what to do,
managed to drive the car home, started it
up the next day & the car in Nuetral was
very juddery tried to drive & it drove & it
sounded load, still with the ESP on
constantly & flashing ECL. The car has
literally has no pull to it ... could it be my
head gasket as that what the garage have
said but when I spoke to Audi over the
phone they said it sounds like something
to do with the sensor or something like that
..... I'm going out of my mind & need a little
help ... please please if anyone could help
me out & point me in the right direction..

The car is at the garage now awaiting a
test
It's already had a diagnostic test & it's
saying a fault with number 3 or something.

No idea ..

I'm worried