[AbRASiON]

If you delete the scrobble on trakt.tv, but leave them marked as watched in XBMC, on next sync (if configured) it will re-mark them as seen on trakt.tv with the date/time. Movies are the only media that pass a date/time when being marked as seen.

If you don't want it to, you need to unmark them in XBMC and delete them in trakt.tv.

I've tried this tonight, we'll see how we go.
Don't know why trakt marked those particular films and episodes as watched, I'd only very very quickly skipped through them to check, normally it insists on 80% viewing before marking (yes, it's set in the options)
Fingers crossed they finally come off my trakt list.

I'm not 1000% sure but it does seem this has finally fixed the problem for me, thank you v.much.
Hopefully trakt won't mark shows I've merely skipped through as watched again in future.