Audi A4 convertible 2007

Hi the emissions control light has been on in my Audi A4 2.0 TDI convertible '07 plate for a while now - I have had it looked at twice and both times it was turned off but as soon as I reverse park the car it would come back on. Yesterday the diesel particulate filter warning lamp came on - so today I went out to take the car for a good run as the manual says to do, but after about 20mins driving the glow plug light started flashing and the car lost all power so we have to 'limp' the 12 or so miles home. Any idea what the problem is and what the cost may be to fix. Thanks.