So I've finally got round to putting it up in the air and I've noticed a few bits about the car which is obviously where they have saved money. But the bit that I want to change the most is there is no under tray.

 

has anyone put an undertray on one before what did you use? I'm thinking a mk7 2013+ diesel fiesta one should fit the best with a bit of fettleing? 

Any tips or help would be appreciated 

Just one thing to consider and that's that an undertray could potentially block airflow to vital components and cause them to overheat, being everything is more heightened in the ST and the car will have been tested for reliability as such.

Also the ST has a 6 speed box, vs the 5 speed on the normal Fiestas, which might make a difference to any fitment.

I've purchased a diesel undertray for my 1.0 but haven't fitted it yet. There are others on this and other forums who have made them fit successfully - hence my purchase. 

Lots of guys on US forums running full aluminium undertrays without heat issues. 

Cheers, I'll have a look for one if you can remember can you let me know how much of a pain it was to fit 

It does fit you just need the correct fixings when you buy the tray, the tray is about £80