Thank you, I'll take the seat away and I'll know.............Look at the drawing here: http://www.partzilla.com/parts/search/Honda/Motorcycle/2001/ST1100+AC/CARBURETOR+++COMPONENT+PARTS/parts.html
Now look at the float assembly. Look above the float assembly to the small box numbered 1.
You see the needle, the seat, a gasket and that small plastic part which is a last chance filter.
This is my best guess. I could be wrong. But you get to find out!
You were right, Mcreviver, it's a very small mesh filter, the old ones were perfectly clean so I decided to keep them on.Thank you, I'll take the seat away and I'll know.............