Air filter - Ripspeed universal filter...for the money you can't beat them. Better than K&N I can assure you.
Exhaust - I'm having trouble finding manufacturers that make full systems for the 220SDi not sure about other L-Series cars. Have found I think Laser and Sebring that do just backboxes for them but they are £250ish, which is about the cost of a full powerflow system.
I've got an ITG straight replacement filter in my L series powered 200, I know Rover Ron's got one in his motor too. They seem to make a little difference with response and perhaps a slight improvement with acceleration but nothing major. However, in conjunction with a decat they do make a significant difference, I'd highly recommend either getting a decat pipe made up or knocking out the cat from the existing catalytic converter on your car. This will make a noticeable difference, far more revvy and very noticeable in 1st and 2nd gear and below 2300rpm (which was the point where the turbo boost came in on my standard SDI - it's now on from about 1300rpm with a tunit box and decat)
As far as I know, the airbox on the 200 (and 400?) derv isn't that restrictive anyway, besides which diesels suck in more air than they need for combustion purposes so you're not going to get huge gains this way, if you want serious power and performance increase you're going to need to get more fuel in there at some point (e.g. chip/tuning box etc.)
I got my ITG air filter from www.motorsportworld.co.uk, the reference number is ITG WB 290 and cost about £35. Like I say, it won't set the world on fire but it all helps, and with a decat you'll really feel the difference