Got Fiesta 1.2 08 reg guys and the past couple of months since the weather has been freezing I've had this engine tapping/rattling noise on a cold start if I drive straight away but if I leave it 'warm' up couple of minutes there is no said noise. The car was serviced is January and the mechanic reckons the engine is fine but what could this noise be?

Just the cold rattling its bones. It takes longer to warm up and close the tolerances between parts. On a cold morning, one of my other cars sounds like a diesel and shakes as though it is about to disintegrate, spreading parts all over town. Fifteen minutes later, it is silken smooth all the way to the 7,250RPM redline. Your mechanic  is quite right.

I've just never had a car that would make this noise on a cold start lol

i've got a 1.4 tdci and when its cold it sounds horrendous as if its been running with no oil and everything is smashing together, few mins of driving it all warms up and sounds like a new engine. granted its a diesel so a little more noisy but same procedure the oil needs to warm to get moving enough to get where it needs to be. if the car is driving fine and the oil levels are good with no warning lights etc. then i wouldnt worry about it.

You may just want to ensure the correct grade of oil is being used at the next service.

It's probably a bit thick when cold and it takes time for the hydraulic lifters to get oil and adjust

At fifteen years old, the pump will be a bit slow and all the tolerances, tappet bores for instance will be worn so it will be noisier than a new car. Note I wrote worn, not worn out.