<p>Thanks for the replies, I didn't think it had any function but better safe than sorry.<br>
Wow, $100! That's £64 or so, about 15% of what I paid for the lens. I think I'll do without.<br>
Edward, thankfully it isn't the knurled rubber focusing ring but the plastic bit of trim that sits next to it (towards the mount).<br>
Again, many thanks to you all, Peter</p>