I bought one a couple om months ago - they are great. They will fit the end of my number four section (Maver Respect) and it does fit the 16 meter extension. I have seen them several times in local outlets but any Seymo dealer should sell them.
Great info, great guys, great fun, thats Maggot Drowners
Here's a link to http://www.hopkinsholloway.co.uk/ who are the manufacturers of Seymo products.
They have the Expander Skid Bung on the site with a list of Dealers by clicking the 'More' button next to the product (Here)
Hiya MrC, bought a seymo expander bung about 4/5 months ago, infact i did report on here about it recomending them because you only needed one bung,
it adjusts from 35mm to 50mm. it works a treat the only fault i can find with it is if you fish the pole by sitting on it the bung can become cockled, you'll soon know what i mean if you get one!