I know labouring an engine is bad for it, but how does this work with the extra torque available low down in the ST? Say you're going quite slowly in 4th or 5th then floor it, does having the extra power there simply put a lot more load on the engine making it even worse than labouring a NA car, or does the extra power mean it isn't actually labouring? I find myself not bothering to change down sometimes as the car handles it without, but I can't help wondering if it's not a good thing to do as the revs have to build a bit before the acceleration kicks in.

I'm not sure what classes as labouring / not labouring, but I try to not go below 1500 rpm except in 1st and 2nd. I've always assumed that if the engine can get you moving easily without the kind of vibrations that are normally associated with labouring the engine, then you're ok.

If you go too slow in too high a gear as you have found out it will not accelerate until the revs pick up.

The Fiesta also has an anti-stall feature and this may make you go too fast in say a 30 limit.

You should also be aware that labouring an engine also knocks the small ends out (eventually)