Can anyone help identify this "retro" muddyfox frame?
OK I've searched a lot and can't find anything that matches.  Muddyfox made a lot of bikes over time, so this is less of an answer and more of a dump of observations:

Points to note 


Overall the bike looks "thick" - the chainstays are chunky, as are the seatstays.  
Having trackends for rear dropouts is quite unusual on a MTB, but less unusual on a BMX.
The frame reinforcing gussets behind the head tube are unusual for a MTB.
The front suspension looks newer than mid 2000 because it lacks the stanchion boots which were common on earlier forks.
Paint on forks looks identical to frame so its likely to be original, making the bike a 26" hardtail
downtube


From a distance, it looks like a BMX, with a relatively low seatpost clamp.

I bet there's no brazeon mount for a front derailleur - and it may help to look closely for any paint marks where a bandon derailleur might have mounted.  If nothing found, I'd say this was a 1x setup from new.

At the rear, you have an electric motor now.  Was there any indication of a rear derailleur at any point ?  If not, this suggests the bike was a single speed OR it was fitted with an internally geared hub from new, both of which need trackends to set the chain/belt tension.

UPSHOT I think its a mid 2000 bigger kid's frame, designed to "evoke" the BMX, so possibly intended for teenage boys who have outgrown their 20" BMX.   

It might even be called a 26" BMX in the right light.