What's the legal situation with regard to posting links to reputable news outlets
If you repost defamatory information gleaned from other sources you can be sued. If you link to other sources the legal situation is less clear but it is unlikely you would be sued (unless, of course, you had set up the other source yourself).
I would love to see Mn take a proactive stance on this and stand up for MN users freedom to discuss the issues and evidence put out by Samaritan's Purse
The problem for Mumsnet is that once they receive a notice from an individual or organisation claiming that a post is defamatory they have follow the process laid down by the Defamation (Operators of Websites) Regulations 2013. The first thing they must do is send you notice of the complaint and give you the options of having your post deleted or providing your full name and address. If Mumsnet fail to follow the required process they can be sued for libel and I suspect their insurers would tell them that they were uninsured. So standing up for your freedom could mean no more Mumsnet. I suspect that isn't what you want.
Note that if you provide your full name and address, tell Mumsnet you don't want that information released to the complainant and say that you don't want your post removed Mumsnet can safely leave your post in place. However, it is then open to the complainant to get a court order to force Mumsnet to disclose your details so that the complainant can then take action against you.
surely a court order wouldn't be granted unless they could prove that what you had said was illegal
The complainant would not have to prove that your post was libellous to get a court order. They would simply have to convince the court that there was an arguable case and that they should be allowed to bring that case.
For those who say this is an attack on freedom, the central question is whether the internet should grant you the freedom to libel someone anonymously and not give them any comeback. Unfortunately a lot of internet forums including Mumsnet see a lot of posts that are potentially libellous. The poster isn't necessarily posting maliciously (although in some cases they clearly are) but that doesn't alter the fact that the post is libellous. Should we really not allow the person being libelled any comeback? A lot of the comment on this thread is fuelled by the fact that many people seem to disapprove of Samaritan's Purse. Would people feel differently if the person being libelled was someone they admired?