Yes, I got timeout, but the important part was getting the right IP address... There's a chance of some error in the way data gets to your ISP from the Mumsnet server (213.161.73.140).
If you imagine the internet connection to be like a set of roads (data pipes) linked to roundabouts (routers) then when you request a web page your PC first does a lookup to translate www.mumsnet.com into an IP and then sends the request out for the page to 213.161.73.140 (and at every router on the way, the IP number is used to decide where to send the request). Once a request reaches a web server, the reply is sent back to your IP address (allocated by your ISP).
It's possible that somewhere between the two places a router is set up incorrectly. Imagine a motorbike courier starting at Glasgow and being told to get data from Mumsnet in London.
S/he leaves Glasgow, going south, to Carlisle, then south again to Manchester, and south again to Birmingham. From Birmingham s/he goes on to Oxford and then via Reading to London. After arriving at the web server, the 'return address' in Glasgow is given and the rider starts off going west to Reading, then on to Oxford, and on to Birmingham.
Unfortunately at Birmingham someone has messed up the sign and has put an arrow pointing south-west for Glasgow, so the rider goes south-west and when s/he reaches Bristol, finds a sign pointing back to Birmingham (and once there, back to Bristol).
Eventually the rider runs out of fuel and is flattened by a juggernaut. OK, that bit doesn't happen, but there is a limit on the number of routers (roundabouts) which can be used to stop an endless loop like this, and the data never gets back to the place where it was requested. So the office (ISP) in Glasgow just waits and waits, and then tells the client, sorry your data went missing. (in fact your PC decides it has waited long enough and reports the problem).
Now, although I've described an error at Birmingham affecting the data coming back, there could equally be a problem with data going to the web server (eg Birmingham signposting London as south-west).
Sometimes a problem affects just one ISP, and sometimes it affects lots of people. You didn't mention which ISP you use, but it might be worth posting on Talk/site_stuff with your situation. We know your DNS lookup is OK, but the technical staff may well ask you which ISP you are using, and perhaps also ask for your IP address.
You can find that out quite easily if you visit ShowIP.Net it will give your specific IP address. That may help the support guys to check whether there's a problem somewhere in the 'local' network (where the server is) or not.
I have in the past found ways around a problem (there's some software from www.http-tunnel.com which allows you to have free access via their service)... the idea is that your web traffic first goes to their server and then their IP address is used for the next part of the journey for your data to get to/from the web server. It's slow (because they offer free access really as a 'test drive', and clunky, but does work, just not something you'd want to use forever). It does, however, prove the web server is up and running (which you already know via your phone) !