An UPDATE for people who helped me above:
I found that in my internet options for LAN the "Automatically detect settings" box had been checked. Not sure how that happened but on the Google forums I found a thread saying uncheck it and I did. I also changed to using Google's DNS servers as people here recommended.
The slow connections problem seems to have gone now, so thanks for that.
I find that sometimes the Plusnet wireless drops and I have to manually reconnect. A few times I have seen not just my Plusnet wireless but also most of the neighbourhood wirelesses also disappear, and I have to turn wireless off and on in Windows to get them back. Not sure what's happening there but things are better than before at least.
Edited by deleted (Mon 01-Jun-15 21:24:05)