I reported this earlier and have been advised that there are some ongoing networking problems at the London Internet Exchange (LINX).
If your ISP is routing to the DC datacenter through LINX then you will be experiencing site drop-outs and slow responses. Some people won't be affected if their ISP is taking a different route to the DC datacentre in Maidenhead.
Not a lot we can do really - it's a UK internet backbone problem.
http://www.theregister.co.uk/2009/12/10/linx_failure/