Re: no proof
I'm a Plusnet customer (since I left Pipex, many years ago).
I use one of the slightly complex schemes you suggest, ie email@example.com
So for example for registering with Pixmania in 2011 I'd have used
That specific example is chosen because the only address leak I've ever had came via Pixmania in the days it was a Dixons Stores Group company (did they sell it yet?).
The jury may be still out for Plusnet, but for me and Pixmania, the case is long since proven and Poxmania were guilty as charged.