OK, so I was nearly too embarrassed to post this. I've got hundreds of hours on KSP, but I've been away from the game for about a year. I tried to construct a basic rocket to just test out what had changed, and couldn't get the engine to attach to the bottom of the fuel tank. I'm using the latest version (experimental?). Everything else seemed to attach as normal. I spent over half an hour trying to do it, using different combination of parts. Has something changed here?