Switched to plusnet & can't access a lot of websites please help

[#1]

Switching to plusnet has been an utter nightmare & i'm starting to regret choosing plusnet...
I chose to switch to plusnet after being with Sky for almost 5 years and having connection issues with them for the past few months so decided to upgrade to fibre with plusnet in the hopes I would get a better service.
But after ordering and paying for the first month of fibre the change over date came and went without a bt engineer arriving or any explanation from plusnet, So I call to find out whats going on just to find out that plusnet decided to change my order to adsl broadband, without telling me even though they have already sent me the fibre router, so once they connected the adsl broadband I had to then wait for someone to realise they have messed up and had to get them to talk me through changing the settings on the router to an adsl router instead of fibre.
I have pretty good speeds at the moment but for some reason I can not access quite a few websites/apps.
I work from home and need to use yahoo messenger to communicate with my colleagues but I cant log into it now, also cant access yahoo mail, twitter and a LOT of other websites, I have connected my laptop to a neighbours internet and also connected to my mums internet and all those websites work fine on both of those networks but as soon as i connect to plusnet at home again I cant use any of those websites.
When I spoke to someone at plusnet yesterday about it all they said was 'there is no reason you cant access those websites' no offer of any help to sort it out 
If anyone has any ideas how to sort this issue out please help me i would be so grateful. After all the problems with switching over I'm ready to pull my hair out 

[#2]

I've been a PN customer for years, but I've always found their DNS servers to be 'iffy' at time, so change these to 8.8.8.8 and 8.8.4.4 if you know how to and see if that improves things.

[#3]

Thank you gingerbloke that worked   I can now get on yahoo mail and messenger still cant get on a lot of other websites but at least I can work without any problems, you have saved me from going completely crazy   Thanks again

[#4]

What are you seeing, specifically, when trying to access these web sites? Need more info to diagnose that problem.

[#5]

You're not the only one with such issues recently. See:
http://community.plus.net/forum/index.php/topic,137249.16.html
Try all the sites you have a problem with on Plusnet connection via a middle man online proxy:
https://hide.me/en/proxy
Do they then work ok? In my case, any site with a problem works via a proxy.
Look up how to do a pathping in a DOS command prompt and you should be able to see where packets are being lost with your connection to the problem sites. In my case, they show 98% loss and above with some Plusnet hops.