No access to some websites

[#1]

You have taken much trouble to find the fault, and for understandable reasons - but I believe incorrectly - have concluded that it cannot be at your end.
I say incorrectly because it is surely absolutely impossible that the fault is at my end because it has been present while using no less than three computers and two routers, one wireless and one not?  Splitters have been changed.    Please tell me if you agree with this conclusion, and if not why not.
Clearly the problem is most unusual, but I very much hope you will take up the challenge to isolate and correct it. There is obviously a very precise electronic fault somewhere, and it is after all preventing me getting what I pay for -  a properly-working broadband connection.  The fault must be somewhere!
N.B. I am using this site because today I am unable to access the Questions page and so cannot comment there.

[#2]

Can you give a few examples of sites you can't access
Is there a common factor across the computers such as Security package
With respect to the questions page - have you inadvertently blocked cookies from Plusnet as that may cause that problem
Have you tried a tracert to a site you can't access

[#3]

most probably a setting in your web browser security setting i would say 

[#4]

Thank you.  The solution to the problem must lie outside my setup because my computer works well on another line, and my router is the second of two (different makes) which have failed on my line.
Websites I cannot access are e.g. British Rail, Paypal and The Times(though I can acess the front page).
The problem has been present for at least two months, and with three computers, two having failed.

Regards
Johneh

[#5]

As your latest computer works on another line can we go back to basics
Can you report the information requested here http://community.plus.net/forum/index.php/topic,96155.0.html
Also are you connecting to the router by wireless or cable
Do you know the reasons for the other two computers failing as losing two in a short time does raise concerns