OK - cancel the question - I just found a useful photo guide on alfa156.net under the tips section.
In summary, the pollen filter on a pre-facelift 156 is located beneath the plastic under the windscreen on the passenger side, so you don't need to remove the wipers, just park them pointing up the windscreen by turning the ignition off in mid travel!
I only removed the screw nearest to the centre of the windscreen and the one next to that. This gives enough clearance to remove the filter. You don't need to remove the screw at the far side of the windscreen - just as well since mine was rusted solid. Pull the plastic away from the windscreen, unhitch the rubber bands holding the filter in place and lift out the filter, noting that the arrows on the front of the filter point towards the ground. Insert the new filter the same way and reverse the procedure.
One problem I had was that when I pulled the plastic shroud up, three of the white clips which are supposed to remain attached to the plastic were left in the bulkhead, making it impossible to press the plastic down again. I removed these with a pair of pliers, re-inserted them into the plastic and was then able to refit the plastic. The pollen filter part itself cost £21 at a main dealer.