The ICO have a complaint form for spam, here.
I was fooled by question 2c which referenced waiting 28 days after asking someone to stop. What I had missed was that question 2a says to skip that if you have no relationship with the sender.
The ICO sent a helpful response to my query and explained this. I am impressed.
They also stated, very very clearly :-
"Firstly, I would confirm that your
understanding of Regulation 22 is correct; that is, if a company sends
an unsolicited marketing email to an individual subscriber without their
prior consent and/or without satisfying the conditions of the soft
opt-in, then they will be breaching the Privacy and Electronic
You don't get clearer than that - thank you ICO. I will be quoting that in future letters and emails to spammers.