Was driving along the motorway at 70 MPH and out of nowhere the revs dropped to 0 - rolled to a stop on the hard shoulder and tried to restart, ignition lights came on as normal but it wouldn't turn over at all

Had a look under bonnet and F28 in auxiliary fuse box had blown, so I swapped it out for a spare. Engine turned over for a second or so before it blew the replacement fuse and back to square one - ended up getting recovered

From what I can see in the Haynes manual F28 is for the fuel injection ECU - going to hazard a guess that this is likely the culprit as there was a slight electrical burning smell in the cabin

Any advice on further diagnosing/potentially resolving this issue? Is it more likely to be the ECU or a wire somewhere that has frayed over time and shorted to the body somewhere?

Don't want to send her to the scrapyard, other than this she's a tidy example with 44k on the clock and you don't see many MK4s about these days. My local garage aren't interested in taking the job so going to have to attempt to sort it myself

Mk4 Fiesta is a bit early for me so I'm not sure whether there are any common faults.

I would start by sending the ECU away testing if you can't see any obvious faults.  If that comes back fine then I'd move on to wiring checks.

Disconnect the fuel injection ECU. Replace the fuse and see if it blows again. If yes, then you have a wiring fault, if no then it is likely to be the module at fault.

The attached should be better than Haynes!

Fiesta 96-99 F28.pdf