Your obviously doing a DNS resolution properly, its then not allowing access to the site. I would say its a Vodafone issue blocking the site. You could try a Trace Route to see where you time out.
From a command prompt type in:-
tracert
www.airgunforum.co.uk
Tracing route to airgunforum.co.uk [109.75.172.146]
over a maximum of 30 hops:
1 <1 ms <1 ms <1 ms bthub [192.168.1.254]
2 4 ms 4 ms 4 ms 172.16.18.143
3 * * * Request timed out.
4 13 ms 13 ms 13 ms 31.55.186.184
5 12 ms 13 ms 14 ms core4-hu0-7-0-8.faraday.ukcore.bt.net [213.121.192.60]
6 12 ms 13 ms 13 ms peer3-et7-0-7.redbus.ukcore.bt.net [194.72.16.94]
7 13 ms 13 ms 13 ms ge-0-1.linx.lon.nodefour.net [195.66.224.135]
8 15 ms 15 ms 17 ms mx02-xe0.0.1-nht.dc4.nodefour.net [109.234.203.245]
9 15 ms 18 ms 15 ms 77.111.212.79
10 18 ms 18 ms 18 ms mx01-xe2.0.2-wkf.dc3.nodefour.net [77.111.212.13]
11 19 ms 18 ms 18 ms 77.111.217.13
12 18 ms 18 ms 18 ms agfuk.airgunforum.co.uk [109.75.172.146]
Trace complete.
This will tell you if you are leaving the Vodafone network to get to the website or if you are getting blocked on their network