Just a little update on this - I emailed MK as suggested above (the extension is listed on their website for £40 as Ozzy said) and had no reply and they didn't pick up the phone when I tried to call, so won't be using them.
I went on to eBay and managed to find a new old stock lower column that looks more 'extension friendly' than mine:
Fingers crossed this will work with the thick walled tube I have.
Cheers again for the help guys