mines with esure.
With yours being a diesel, i suggest doing more research with regards the air filter idea. On several other forums i participate in, the general consensus (this is with mondeos bare in mind) is that diesels dont get any improvement with different filters, they usualy recommend to stay with the paper ones and replace them every six months or with the oil change. The only benefit people have found with the different filters such as k &n or pipercross, is the sound, no power gains really.
There are also similar results with induction kits, they will only make enough difference to warrant getting them if you can heat sheild them sufficiently, but due to the lack of space in modern engine bays, this can be difficult.
Id suggest holding fire with ay modding till your back on your feet and money is a little easier to come by. It can be very addictive once you start.
I agree in general with this - K&N panel filters, cone filters and other "performance" filters / intakes CAI(S) (cool air intake (systems)) are much hyped and often offer zero or little power gain, and if it is a cone filter under the bonnet, power loss due to drawing in warm/ hot air, K&Ns can let through fine dust particles that can damage the turbo compressor wheel (ive seen pictures of the leading edge being worn down) cause bore wear and contaminate the engine oil, the oil on K&N filters can contaminate the MAF sensor - stock paper filters are better at filtering out dust and tiny grit particles, and cannot contaminate the MAF
Its a big, complicated subject and ive gathered a lot of data on it, there seems to be a "pro" and "anti" K&N/ aftermarket filter following. . At the moment i run a stock paper filter (with gas-flowed stock airbox with large diameter intake) and change it about every 6 months. but may be doing some of my own research on this in the search for more power/ torque (and am going to go into it with an open mind) - but it looks like any improvement in airflow/ power, is liable to reduce filtering efficiency (potentially allowing fine dust into the engine)
For most cars, replacing the filter for a K&N makes no difference at all, and you only notice a difference because the old, stock paper filter was clogged/ needing changed, and if it makes a bit more noise it seems more powerful (placibo effect)