Broadband only works on some sites

[#1]

For the last few days my broadband only works on some sites, google, blueroom, screwfix, giffgaff, metoffice etc all work fine, other sites won't load at all ie lloydstsb, the register, roblox teignbridge.gov.uk etc. I presume this is a dns issue, I have raised a ticket but just get the usual stuff the csa reads off their screen

[#2]

Hi Peter,
I'm sorry about that - not the greatest reply to your initial ticket, which we'll pick up.
Regarding the not being able to open those sites, are you able to ping them?
If you can't pick the actual domain name itself, but can ping the IP address of the site, then it would indicate a DNS issue but not loading the site itself, doesn't reveal too much information I'm afraid.

[#3]

When I ping the domain or the IP I get Request timed out ( roblox, lloydstsb ), but google and others that work less than 30ms
Interestingly plusnet, 212.159.9.2 I get request timed out
[edit]
Plus VOIP hasn't worked since Saturday