Is this only affecting Gmail recipients? Are those Gmail recipients aware of a new Gmail policy to dump any mail into spam folders without checking that it's genuine and fully authenticated? TalkTalk Mail is sent fully authenticated.
TalkTalk are aware of Gmail's new policy and have done what they can. What you can do is advise Gmail recipients to select your mail from their Spam folder, select the Report as not spam button and this will tell Gmail to deliver your future messages to the Inboxes of your Gmail recipients.
