Sorry but it IS important to number the scenes. Numbered scenes are necessary for a read through, rhearsal and for reference. If you don't number them, someone else will have to. You should also be indicating time, i.e. when the event is taking place DAY1. [12.30PM], DAY 2. [9.00PM] etc.
A Flashback is a separate scene with its own set, characters and dialogue and will have to logged by the director and editor, so will require a title of some kind. You can call it 'FLASHBACK 1.' or 'INSERT 1. Flashback', or 'SCENE 12. INT. SHOP. DAY. FLASHBACK', where it follows on from SCENE 11 etc., but you must call it something.
Martin, I think your first effort is more-or-less there, you just need to indicate a transition at the end of the flashback, as AJP has said – either 'CUT TO.' or 'MIX TO' if you want a disolve rather than a cut.
'CONTINUOUS' is a direction for when action follows on in time, so you would not use this for a flashback. Use this when a character walks down a corridor and opens a door in one scene and is then shown entering a room in the next. 'Continuous' indicates to the reader that the scene follows on with no delay in time. (The time in the character's world).
To interrupt a scene, if for example you used the 'INSERT 1' direction, use the same scene number and CONTD. (CONTINUED).
Hope this helps.