It was only a 'cheapie' one, £30, from a local auto spares shop. Plus £10 for a breather for the crankcase ciculation (the 'mini' filter)
It was very easy to fit, no modification whatsoever, took ten mins.
The filter had a 60mm neck, but doesn't bolt on, so what I did was:
1) Took off all the old air box.
2) Took off the pipe that brings air in from the duct in the bumper (it is pre-bent into the correct shape I needed! and it was 50mm - perfect)
3)using a jubilee, and after taking off the rubber and metal gaskets from the top of the carb, i placed the correct end of the pipe (u will know because of the way it is bent to allow filter placement in the engine bay) at fastened it - perfect fit, the diam. of the carb top is 50mm.
4)Using a piece of soild old 50mm tube I had around, of very short length, say 50mm, attach one end to the pipe and the other attach to the filter using it's prefitted jubilee clip.
5) Now, you just have to take off the rear cover to the left headlight
6) The filter positions itself perfectly snugly in the space to the bottom left of the engine compartment, behind the headlight. It even fitted in my car with the ecu in the way which is fixed there
7)The inlet pipe is already there from the bumper, upgrade it a bit - perhaps a bit of pipe to direct airflow, etc.
8) Fit the breather filter to the cranky pipe and I just used some small malleable metal wire to secure the filters - all you need.
9) Prepare to be deafened.
hope this helps