I don't know the model but it's 1985 or 86 - more likely 86.
All 87 Muddy Fox models had U-brakes, 88 went back to canti but your components are too old for it to be an 88 bike. So it's pre 87.
The Muddy Fox / Silver Fox thing came later I'm pretty sure, the 'MKII' version of the company after the original went bust.
Terry...