Sourcing the part

This task is the replacement of the stock air filter (which I believe is Lucas brand) with a performance filter, such as the K&N E-2653 which can be found at several places for around £40 to £45 at the time of writing.  The part can be sourced from here or here, or you could try your luck on eBay. 

Where is the air filter located?

The air filter is located at the back of the engine, close to the bulkhead, under a plastic cover through which the oil filler cap pokes.

The air filter is located at the rear of the engine, beneath a plastic cover

The cover is secured by five hex/Allen bolts.  Three of them are on the top surface of the plastic cover and are easy to get to, but the last two are on the back edge, and are close enough to the bulkhead to make removal awkward.  Thankfully they are captive bolts, so there’s no risk of losing them in the engine bay as you wrestle with them.

The air filter is just visible behind the front wall of the air filter box

Once all five bolts are unscrewed, remove the oil filler cap to allow the air filter cover to be removed.  Both engine covers and the air duct have been removed in the above photo for illustrative purposes, but this is not necessary for changing the air filter – removal of the air filter cover (the rear engine cover) is sufficient.  With the filter cover removed and before removing the filter element, replace the oil filler cap to prevent dirt and dead flies from the filter element getting in there.  The air filter pops out, and the replacement pops in.  You may well be surprised at how dirty the stock filter is.

The difficult part is replacing the air filter cover.  You would be forgiven for thinking that it feels like the replacement filter element is too big for the filter box; indeed, if you replace the original element the cover seems to snap back on with no problems.  The new element, however, needs careful trial-and-error realignment of the filter cover before the Allen bolts can be fully fastened.