Pollen filter changed! Thank you RUI for the reassurance on where the filter is actually located.
On a UK car, it's located in the drivers footwell. There is 1 T25 Torx screw to remove, and 2 T30's which will allow you to (carefully) pull off the trim, pictured below. The filter is located behind the panel which I have marked with a green arrow in the pic in my previous reply. The best way to get it off is just to get your hand up and pull it towards you.
To get the filter out, I just about got my thumb and index finger in and prised it out, although you could use pliers.
To get the filter back in - persevere. I went above the clutch pedal and put the bottom in, and forced the top in and then straightened it up. It's hard to describe but just be patient and use a bit of common sense.
Not a 5 min job as I anticipated, but certainly a job that most owners should be capable of doing.
Here is the filer I removed:
It wasn't Lancia/FIAT/Alfa part - it was marked "Denso", so I suppose it's been changed at some stage in the 25K the car had covered before I bought it. I got a genuine part on eBay for £8 delivered for info, and it is snow white in colour to give you an idea of just how bad the one I removed is.