Disclaimer: This page details my personal DIY experience. I have no qualifications to do this work. If you carry out any DIY based on information on this page you do so at your own risk and I accept no responsibilty for any negative outcomes.
This is how I replaced the spark plugs on my 1.4l Mazda 2 (05 plate). I understand the engine is basically the same as some Fiesta models (e.g. Ford Fiesta 1.4 Zetec/Duratec). The problem is accessing the spark plugs in the first place. This is what you see when you open the bonnet.
To get to the spark plugs you need to remove the air filter housing. Obviously the two bolts need to be undone. The large hose to the right can be separated from the housing by twisting away from you (clockwise) and sliding out. There is a small hose to the left that simply pulls out (circled below).
The housing is now loose but still attached at the back. After a lot of web searching I found a page about changing the air filter on a Ford Fiesta Flame 04 reg. The page says that the housing is held to two posts at the back by rather grippy rubber washers and it is difficult to lever the housing off. It says that if you can't get it off you can dismantle the housing to replace the air filter. This is not an option if you want to replace the spark plugs as you have to get the housing off.
I levered the housing off using a lever (piece of wood) and wedge (screwdriver). If you lift up the front of the air filter housing and look underneath you can see the two posts it is attached to. I pushed the piece of wood between the housing and the engine so that it was next to one of the posts. I then lifted the end of the wood so that I could get the screwdriver between the wood and the engine block. Finally, I pushed down on the end of the wood to lever the housing off the post. I repeated this process for the other post. The piece of wood I used was quite flexible so it was quite difficult to get the housing off. I am sure using more appropriate tools (especially a solid lever) would have made it easier.
Once the housing has been levered off the two posts (circled below) it can be moved to the side to reveal the HT leads. I didn't need to diconnected the large hose on the left of the housing. You then simply pull off the HT leads and replace the spark plugs. I used NGK TR5A-10