my speedo cable broke the other day and since then the engine check light comes on sometimes and the car is popping and trying to cut out when i come to a junction could this all be down to the cable being broke.

 

What year car?

If the speedo cable breaks or there is a problem with it the ecu will go into limp home mode bringing up the CEL and limit you to 0.5bar boost and rev upto 4500 rpm (this is what happens on early classics possibly all imprezas but i don't know that for sure)

 

i had a simular problem on my 96 sti . replace cable and reset the ecu by disconecting the the battery for 4 hours or so should do the job