When a flight plan is submitted i was under the impression it had to be activated when the aircraft departed. It appears this is not necessary under the Sky Demon system. Is the flight plan then automatically activated at the time stated?

You still need to ask your departure airfield, or an enroute service, to activate your flightplan when you take off.

microflight - the best way to understand the system is to visualise flightplans simply as emails on a dedicated aviation network (sounds crude but far away from reality as this system originates from Telex days); you "filing" it just sends this formatted "email" to your departure and arrival airports plus some agencies in the back
in fact a VFR flightplan isn't stored anywhere outside SkyDemon, like an email it's "fire and forget" (the IR system for European CAS is different, but won't apply to most people here); so there's nothing you could "activate" later; "activating" the flightplan actually means your departure airport (or the first FIS controller you talk to) sends a "departure message" to the arrival airport, so they know roughly when to expect you (and when to start S&R if you don't show up)