I see your Colchester, well don't know if he is on here but theres a guy on most Ford forums called markymark from Chelmsford and hes a Ford breaker and would be good for getting the parts, might even fit it for a good price if you ask but guessing your car has ABS? If so then its a straight forward swap just need complete engine and gearbox plus loom, mounts few bits like that from the ST and engine is simple swap if you have a bit of know how as the good thing about the mk6 over the other Fiestas was it was the first i believe to allow you to take the front reinforcer off to remove the engine, not the normal up and over or down and under.

If your car doesn't have ABS then you will have problems fitting the engine as the ST engine doesnt seam to want to work unless it detects the ABS. Then you will need aftermarket ECU IIRC.