Thank you for your very prompt reply.
I had already deactivated the Widget disable. In fact, I realised that as long as I only put widgets in the Widget Sidebar (the first option) they would only appear in posts where I used a Sidebar Left template – or at least that’s what seems to be the case. Therefore no plugin needed.
But I still hadn’t cured the problem of this rule
#content{width:60%;padding:4%;float:left}
jumping in and forcing a left float when I wanted a right float, despite targetting the float on the required post template. Everytime I inspected the code there it was.
I’ve fixed it now by changing the overarching rule to float:right and the page templates to float:left. So far, it’s OK – don’t seem to have broken the Internet. :-)