Is overblocking defamatory?

If a user of an ISP tries to visit a website controlled by an identifiable legal person, which does not contain any pornography, but the ISP instead serves them a block page informing them that the website does contain pornography, has the person controlling the website been libeled by the ISP? (All three parties located in the UK.)

(If you think Cameron's latest wheeze won't lead to overblocking then you haven't been paying attention to the existing implementations.)