This post is a call for help from somebody with networking knowledge. I was not able to get any help from the BT support line but would like to share the issue. So, my symptoms are that although I am connected by wireless to the router I am unable to gain access to the internet. This is confirmed with an exclamation mark against the wifi icon on my Android devices and an asterisk on Windows. The condition is also random as the router seems to allow access on reconnection but then remove it again when I next connect.
Further checks identified a further indication and this is that the DHCP allocated IP address contains a 0 (zero) in the 3rd octet. For example 192.168.0.63. This 3rd octet should be a 1 (one) and when it is the device works ok. So, it seems that the router is randomly issuing these rogue IP addresses. Hes anybody else had this issue and if so how was it fixed? FYI. Hard wired devices don't appear to suffer from this issue.