If the rad outlet is cold after a decent run, but the engine is overheating, then it's not doing its job properly, which is down to one of three things:
1. The rad is blocked (either internally or it's got a plastic bag / excessive road muck stuck on the front of it!!)
2. The stat has failed closed
3. The water pump is knackered
The only reason the heater helps things is it is taking the place of the non functioning radiator (which would rather suggest that the water pump is OK) - the rad will have plenty of capacity to cool the engine in the UK climate on its own without the heater. A 2011 car would almost certainly have come with long life coolant, so it seems unlikely that the rad is blocked internally.
I would:
Take the stat out and test it in a pan of hot water - if it fails, fit a new one. If it passes, then run the engine and take off a bleed screw cap - water should shoot out if the pump is OK.
If it does, then you are down to the rad - there was a thread recently about a funny clonking noise on a Focus which turned out to be a radiator flap - is there one on a Fiesta, and has it failed closed?
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