Pipex confirmed that they are throttling P2P usage. Despite what is said on http://www.pipex.net/legal/heavy-usage.php, this throttling is in force during off-peak times as well i.e. all the time!

The customer services person I spoke to sounded so pathetic trying to justify th e policy, I had to let him off the hook - poor guy! But I don't think I'll stay with Pipex out of pity.