Are you saying you do use them? Then why not for Plusnet? If you aren't, how do you know?
I think they probably disappear at Financial Year end or something. In my 30 or so D/Ds and SOs there is only one showing as Cancelled, with a last taken date of 15 Aug 2010.
That one is interesting. It is O2, clearly for my mobile phone, but there is a replacement for it with a different Reference but the same amount. I don't think I shall bother investigating.
I did use DDs quite a lot, until I hit a problem with one mandate that I had completed which never reached my bank.
It was then that I was told "we never send mandates to the bank - it's an automatic system, if there's a problem the bank will let us know". What !!!
So I've based all my comments in this thread on the assumption that this was the general practice.
From your comments and those of other posters, I'm clearly wrong in my assumption, and I realise now that I should have qualified my comments with the phrase "in my experience". Apologies for not having done so.
"I've seen the wonder of the world, its beauty and its power, and the shapes of things, their colours, lights and shades. Look ye also while life lasts."