New cap from Ford is under 3 quid so start there and as has been said clean all your brether system up. If it still does it I would be thinking about blow by on the piston rings pressurising the crankcase but eliminate the cheap fix options first.

Mend that pipe as well while you are at Ford. The pipe is still available and costs about £30 and ensure you use proper pipe clips rather than jubilee clips as you'll be pissed if you split one like I did and end up with Mbil1 all over the drive