Thanks for the advice, have changed DNS as advised above but to no effect so have left it on automatic.
Here's a solution (probably not the best solution but at least web site access is now not erratic): -
Opened "Dell Wireless WLAN Card Utility" and under the "Wireless Networks" tab un-ticked the box: "Let this utility manage your wireless connection".
The status, which was previously stuck on "Authenticating" is now stuck on "Unknown" but at least it now works!
Am I right in concluding that two bits of software were previously trying to control the wireless access?