Mountfield HP-SP470 CUT OFF?

I have just been gifted this lawnmower, the only problem there seems to be is that once started it won't turn off when I release the brake lever, I think there must be something that is part of the brake lever linkage that has broken or come adrift. The only way I can stop it is to pull the spark plug connection off, other than that it runs fine, any help would be appreciated.
Thank you