With the help of Apple Support this problem seems to be cured.
Here is a summary of the steps that they took me through:-
Click on Gear Wheel beside +- at bottom of list of network services, select "Set Service Order" and check that WiFi is at the top of the list.
Click on Advanced and select DNS tab. Click on + at bottom of DNS Servers List and enter 188.8.131.52
In WiFi tab check that your preferred network is at the top of the list.
Apply any changes that have been made and switch off.
Reset SMC and NVRAM/PRAM
Check that there are no channel conflicts with other wifi networks.