PlusNet seem to have taken the commercial decision by building market share by extensive advertising and at the same time going cheap. It would appear that one of the consequences is that they've failed to invest enough money in customer support. I don't think anyone has great confidence in those customer surveys which give PlusNet a best service award, they'd need independent verification/audit to be credible. Unfortunately television and radio advertising will take advantage of the situation.
With PlusNet you are buying an "adequate" service which is reliable in the majority of cases, but if things go wrong it can be slow to fix or difficult to get fast access to the support team.
You can get good service and customer support from an ISP, but you have to pay more.
Now Zen, but a +Net residue.