I have noticed that sometimes the BT’s DNS servers go down due to this i have done some hunting around and found some backup servers which you can use.
nameserver 194.72.9.38 # ns8.bt.net
nameserver 194.74.65.69 # ns7.bt.net
nameserver 194.72.9.34 # ns5.bt.net
nameserver 194.74.65.68 # ns6.bt.net
nameserver 194.72.0.98 # indnsc60.ukcore.bt.net
nameserver 194.72.0.114 # indnsc61.ukcore.bt.net
nameserver 62.6.40.162 # indnsc70.ukcore.bt.net
nameserver 62.6.40.178 # indnsc71.ukcore.bt.net