just come to start my escos, fual pump runs idle valve clicks adn then i turn it over, cranks and cranks but no firing. arm and disarm again and the moment i turn the key to click 2 an enormous BANG with fire pops out the exhaust

fook me, leave it 5 mins, try again and an even bigger bang with a jet of flame and soot/ash bits after , and this is before the car has even turned over.

what on earth can cause this...?
bear in mind, nothing has changed on the car since it ran perfectly the day before, and the only issue with it at the moment id the rev counter issue with the secs coilpack driver.