I left Plusnet after nine months. They never managed to address the major issue which was intermittent huge latency. In reality I don't think their knowledge of networking and DSL was sufficient for them to even understand the problem, let alone know where to look. It was something to do with upload, as when it was in a bad mood I could trigger the problem with around 90-100K (bits/sec) upload. I'm happy to say that no such problem exists on A&A which doubly confirms it was a Plusnet issue and not exchange or my end.
In the end the relationship with Plusnet just soured to the extent that they were pretty much saying that since I was running faster that "the estimate" I shouldn't be complaining, combined with suggestions of putting me onto a 1meg fixed rate.