Went out in my Felicia this afternoon, turned on the radio, smelt burning and the radio/cassette unit was burnt out. Have a spare one at home, put it in the car, turned it on and it burnt out as well! Visible smoke this time as well.

Electrics aren't my strongest point so what can it be? A relay, alternator, voltage regulator?

Over the past week a fuse has blown for the fan, a headlight bulb has blown and the airbag light has come on. It all points to electrical overcharging too me.

Plus I've noticed prior to the radio burning out it had a background tone like it needed suppressing. And when I rev the car through the gears you can hear a faint whining noise above 3500rpm.

Anyone any ideas?

Yeah, get the electrics checked out. Sounds like the alternator is over-charging, but the only way to know is to test it with a meter or alternator/battery tester; anything else is just supposition.

Get it checked ASAP, I've seen them boil batteries and fry engine ECUs. You may get away with a voltage regulator, which can be fitted without removing the alternator. Quite a common fault on the 1.3, there are some on ebay:

Voltage regulator