When I go over a pot hole or damaged surface and there's a loud harsh bang what is this? Is it the edge of the pot hole compressing the tyre and banging the wheel or is it the suspension? With the roads getting worse every day it seems to happen more and more and I keep worrying I'm damaging the car.

What model of Fiesta?

Front or back?

What wheels/tyres are you running?

Any other noises, for instance over speed humps?

It's a zetec s red edition with standard tyres. It's from the front. It's when I hit a pot hole or really bad bit of road.

The standard tyres are 40 series so there isn't much sidewall to absorb sharp bumps. If you haven't driven car's with low profile tyres before then you might notice this more than usual. A loud bang however could be a problem with your suspension. What Year is the car?

2015

I get the same noise with my mk6.5 zetec s. Probably due to the low profile tires and stiff suspension. I just try to avoid any deep pot holes I can. I hope that bang sound isn't the ground pushing up the tire to the rim, I already have an advisory on the MOT saying the front wheels are slightly mishapen. 

I don't know why they stuck with such low profile tyres for the mk7, they were awful on the Mk6, and one of the reasons I won't buy a mk7.  I'm used to just rolling over speed bumps at 20mph with actual tyres, thought I'd burst a tyre first time I did it in the ST150!  

If you're frequently hitting potholes and speed bumps it may have caused a suspension issue if it's a new noise.  The tyre shouldn't completely flatten, they're extra load so designed to take some abuse, but that does mean it puts more force through the rim rather than absorbing it, the wheels do bend easily as well.

Best advice is to avoid pot holes really!!  Even with meaty tyres I cringe if I can't avoid a pot hole... 

Take it back to the dealer if its a 2015 as its still under warranty

They have cardboard for bushes on the front anti-sway bar, and the top hats are also a weak point. My 2014 has one or both on the way out.