403 errors from travel sites

[#1]

I needed to check flight prices early this morning (14 Dec 2011 03:00-ish), so I Googled for 'cheap flights'. Some of the sites returned in my search (e.g. travelsupermarket.com and netflights.com) gave 403 errors. I tried again via a couple of proxies and successfully connected but couldn't use the sites because of Javascript issues. So I pulled out my laptop and tried again using a Vodafone mobile broadband dongle to connect to the Internet and had no problems connecting to all the travel sites for which I got 403 errors when connecting via Plusnet.
I have a strong suspicion that those sites are actively blocking traffic from Plusnet (or at least, from my PN IP address). Has anyone else noticed something similar? Any ideas on what's going on here?
TIA,
Geoff

[#2]

http://www.travelsupermarket.com/ loads fine for me (using Plusnet's DNS servers).

[#3]

It loads fine for me as well and I am also using Plusnet's DNS servers.

[#4]

What is your MTU set to as this can cause problems.

[#5]

My MTU setting is whatever Netgear uses by default. I suspect that this is 1492 bytes as that's the value visible in the greyed-out "Custom MTU size" field, but I have the "Default" option selected.
That said, all those sites now load fine for me, so whatever the cause was it seems to have been transient. As both of you posted well after I reported the incident, you may have been affected without knowing it. I guess it's one to chalk up as a mystery of life, the universe, and everything!