Not sure what I've done but all my sites that I'm usually always logged into such as DS forums are not retaining my details.
Everytime I visit such a site I have to re-enter my username and password.

Any help would be appreciated.

Have you tried: Settings>Safari>Private Browsing>Off ?

I've always found iOS Safari to be awful at remembering passwords.

Some sites it works fine, some it doesn't.

It's the main reason I use an alternate browser on my iPhone & iPad. I used to use Atomic Web Browser, and now use iCab Mobile. Both are better than Safari for saving passwords.