I have traced the problem to a fault at my end.
I had upgraded my Kaspersky AV last week and was sure I had used the news groups soon after but this may not have been the case.
The problem appears to be that the latest version of Kaspersky AV 7.0.1.321 adds NNTP (port 119) and NNTP SSL (port 563) to the network traffic it monitors. Un-checking NNTP (port 119) in the Kaspersky setup solves the problem; merely pausing AV protection will not help
To un-check NNTP open the Kaspersky control panel and go to
Kaspersky>Settings>Traffic Monitoring>Port settings…
You need to restart the newsreader for this to take effect.
The binary news reader I use is NewsLeecher v3.8 – I don’t know if the new build of Kaspersky will affect all news readers in the same way but it probably does. 