This morning my faith in Zen has been restored. They have just reported that they have "found a misconfiguration in the network for your connection and have now rectified this".
My speeds have increased by a factor of 3. ~930Mbps download and ~100Mbps upload on my first few tests.
Needless to say, I shall be keeping a watch for a while to make sure this is maintained. It took a while but we seem to have got there in the end. They must have realised that I wasn't going to give up.

In reply to a post by chriswillsher:

This morning my faith in Zen has been restored.

One swallow doesn't make a Summer 😎