Corsa e vxr not stopping

[Marke32]

I've noticed that when I take my foot off the accelerator say for instance in 2nd doing around 18mph. It slows down very gradually but then just keeps going at around 7mph until I actually brake and stop it. 

Has anyone any ideas what's wrong? 

[Vauxhall Owners Club]

vacuum leak, sticking throttle, many reasons it could be....needs scanning to get a more accurate diagnosis