I've noticed an intermittent issue recently when starting (first winter with the car, so I don't know if the cold weather is causing the issue), but occasionally when starting the engine, it takes a second longer than usual to start; the central locking makes a noise (?), interior light and radio goes off and back on, and then afterwards the average speed/MPG resets on the computer!? Not sure if this is an ignition/alternator/battery issue, or? 

Anyone experienced anything similar? 

Sounds battery related with the other stuff occurring. Mine struggles in the cold found holding the clutch in helps as recommended but still does it. Guess it's a loading issue 

a lady i know at work said  the same thing to me a few days ago about her fiesta , that the starter is slow and it causes a reset of the trip mileage etc. it sounds to me that the battery is in a poor state of charge and saps so much power it causes voltage to drop so low the trip mileage etc resets as if the battery has been disconnected. I said i thought it was a failing battery but could not be sure as she drives such a short distance each day with quckclear windscreen etc on that it might just need charging.  I had nothing with me at work like a voltmeter etc. Then without spaeking to me again she went to Halfords and asked them to check her battery. they siad she needed a new one and sold her one for £59. 

Some might take the view that if you invite a battery retailer to check you battery then they are very likely to say you need a new one as their business objective is to sell a battery to anyone who comes through the door. There are cheaper places to buy a 1250cc fiesta battery.

Anyway, it still seems odd as i would have though a voltage drop so low to cause a reset of mpg etc  would mean that there was no chance of the starter turning the engine at all.

a good rough way to check alternator. have car warmed up. engine running on tickover. swicth everthing on, all lights, heated rear window, heater fan, quickclear windscreen. to get maximum load on battery. now ease the revs up slightly. do the lights go slightly brighter?  if so, this proves the alternator is doing something and is not dead. it's not as technical as using a voltmeter but it's worth trying.