Hi wondering if anyone may be able to help. Yesterday I was pulled over and informed my rear brake lights weren’t working but the 3rd brake light is working fine. 
 

ive changed the bulbs today and the fuse but this hasn’t made any difference can anyone help? 

If the 3rd brake light is working, I would expect the fuse to be ok.

There are two fuse boxes that I am aware of. 1 under the bonnet which I'd easy to see and locate. A 2nd one exists behind the glove box. But as I've already said, I'd expect the fuse to be ok, because the 3rd light is working.

I am wondering whether you have bought the wrong bulbs? A Ford Fiesta to my experience has a two filament bulb.

However, other manufacturers (Peugeot for example), use a single filament for the brake lights.

Ford use the same bulb for rear marker lights and brake lights, hence the two filaments in the bulb. 

Other manufacturers use a bulb each for rear marker lights and brake lights.

I hope this makes sense. Not sure of your knowledge so I hope I've not told you to suck eggs.

"so I hope I've not told you to suck eggs", in the same vein, the twin filament bulbs have staggered bayonet pins sticking out the sides of the metal bits to get them in the correct way. in not located properly the bulb might not be in the holder deep enough to touch the terminals. are the tail lights working?

Hi again. I have a decent knowledge and I understand what you’ve said. Both bulbs I have changed were changed over with stop and tail bulbs I will check the tail lights today for whatever reason never thought to check them 🤦🏼‍♀️  Need to try and sort it today as i’am back at work tonight