Ford KA steering lock andl loss of power

Travelling at approx 60mph on a gently curving left-handed bend.

• First thing noticed was that the Oil and Battery lights came on.
• then realised that the steering had ‘locked’.
• Brakes were gradually applied, left hand indicator applied and slowed right down to a stop (still on the main road).
• Handbrake applied and hazards switched on.
• Turned engine off.
• Turned engine back on after a few seconds and engine started.
• No warning lights, steering ok and so pulled away.

Drove another 5 miles to destination. Parked up etc.
Left destination approx 20 minutes later and drove back towards home.

• Upon slowing down at the traffic lights to turn right into home village, as the turn was being made at approx 5mph, the Oil and Battery lights came on again and steering and acceleration was lost.
• Steering was lost again.
• Handbrake applied and hazards switched on.
• Turned engine off.
• Turned engine back on almost immediately and engine started.
• No warning lights, steering ok and so pulled away.

However, this time the car ‘lurched along’.
Acceleration was applied, changed gear into 2nd – no lurching until clutch was released and then it started lurching again in 2nd.
Acceleration applied and changed into 3rd – no lurching until clutch released and then it started lurching again in 3rd.
Further acceleration applied and changed into 4rd – no lurching until clutch released and then it started lurching again in 4th. By this time, car had managed to get to almost 40mph.
De-celerated down the gears until ready to turn right into our cul de sac. Throughout this, the car still lurched pretty much all the way until pulled on to the driveway and stopped.
It had been rainig on and off quite heavily, so maybe caused by water in the electrics??
A mechanic says the alternator belt needs changing.
Any ideas please?
Thanks