I’ve recently just got the new fiesta ST however it came with the halogen headlights on it. Is there a way I can change the complete unit to the LED version or do I need extra bits to make it work properly? I would have thought it would have been plug and play.

If it is just the bulbs,  then regular LEDs should work direclty if you installed them properly.  Problem with LEDs is that they need something do dissipate the heat,  so you're either stuck with passive heat radiators (looks like fanned out metal foils) or you buy those with electric fans.  Either way,  getting them into the headlight assembly would be the main concern as you need to buy one that fits.  Perhaps the better option to consider would be HIDs.

15 minutes ago, lilbleh said:

If it is just the bulbs,  then regular LEDs should work direclty if you installed them properly.  Problem with LEDs is that they need something do dissipate the heat,  so you're either stuck with passive heat radiators (looks like fanned out metal foils) or you buy those with electric fans.  Either way,  getting them into the headlight assembly would be the main concern as you need to buy one that fits.  Perhaps the better option to consider would be HIDs.

You can't fit HIDs into reflector headlights, you'll blind every person on the road. You need proper projector headlights with the auto leveling and washer features. 

The same goes with most LEDs too, although some of the bigger brands are making some that have a better beam spread and can be suitable for reflectors.

I don't know if it is possible to transfer the whole unit. What i have for mine is these LED bulbs.

https://www.carenhancementsuk.co.uk/collections/performance-mk8-1-0/products/mk8-fiesta-enhanced-edition-led-h7

 

I am sorry but i don't own the MK8 hence i cannot comment too much about it.  However,  based on my experience with the MK7.5,  I would think that the back part of your bulb might not fit your headlight housing,  but feel free to try.  It should not be more difficult than replacing the stock H7 bulbs.  If the back part comes into contact with the rubber or plastic cover,  then do not use the LED replacement.  One thing bad about using LEDs with fans is that there is no way for the heat to escape from the headlight housing and it ends up as the hot air recirculating within your headlight housing and that is not good.  I am still one for HIDs if you ask me.  I have LEDs with the fan on mine but only for the high beam since they are not on all the time. I do agree that you can't put HIDs on non reflector lamps,  but there has been HID bulbs with additional sticker on to simulate projector cutoffs.  I've tried a pair and although they are not 100% as good as projectors,  they do help with the cutoff.