Re: fiesta mk3 air filter
I know K&N do a filter element replacement for the Fiesta Mk3, part no. E-9090 I believe. This is designed to fit in the oval type shape filter housing.
I would imagine your car has injection if it is L reg, so I think it has the same as mine.
Green probably do an air filter element as well (They do one to fit in my Mk1 Fiat Panda!)
It depends what you want the air filter for. It won't make much more noise and the power increase won't be huge, but you can only try, right?