New air filters
I finally got around to looking at the air filters. At quick glance they didn't look too bad inside the cans, but when we got them out they were pretty filthy as you can see.
The original black cans pull apart quite easily with a pair of pliers pulling the face-plate off. However, because the new filters didn't have as much give in them due to them having a metal surround, they were a bit more tricky to get to stick together again. Hopefully having them screwed back onto the carbs will seal them up nicely.
This might give us a touch more responsiveness as we've now improved the airflow to the carbs - it must have felt like it had asthma with that much dirt in the filters. We won't know for a few days as we're in the middle of painting the engine block.