Plusnet have been in touch to say:

"We'll be making a change to block incoming traffic on ports 53, 111, 135, 137, 138, 139, 445, 515, 1080, 1433, 3128, 3306, 6000."

It's all about stopping 'bad' traffic to hacked routers/networks. I'm generally not a fan of ISPs doing this sort of thing because it's only one step away from censorship. However, I suppose the reality is that people aren't especially good at securing their home networks, so quite a few of them are vulnerable to basic attacks.