Many apologies for anyone who failed to reach the forum for up to an hour this afternoon. The problem has been fixed, and everyone should have access within an hour.
Technical description: I updated some DNS data entries, in preparation for moving the forum to a new faster server, but forgot the slightly-convoluted way the site's hostname gets resolved. The CTC DNS zone has a CNAME which points to ctcforum.fonant.com, so that I can set the server's IP without needing to ask the CTC to change their DNS settings. I forgot that the ctcforum.fonant.com hostname was effectively part of the ctc.org.uk zone.