Just incase you need it, here's the link to Burlen's Zenith 26VA page with a useful zoomable exploded diagram - unfortunately it doesn't appear as if they supply new floats for the 26VA, though they do produce them for different types of SU carb. I'm sure the cherished suppliers will be able to supply you with a used float if yours proves to be defective.

http://zenithcarb.co.uk/carbspec/carburettor/spares/id/2841/

Jeff.

Location: Almost but not quite, the far North East of England

Gower and Lee are an alternative supplier of Zenith parts, they may have the bits you need. A google search should find them.
John

Have you recently run low or run out of petrol? If so it could just be dirt.