Flight question - kinda complex.

Hey guys, quick question,
Went to Japan 7 months ago for 3 months, i was originally only going for 2 months but on the day of my flight i was really sick so i ended up not catching the flight and staying another month - i purchased a return flight from Narita to UK.
Due to fly back on 30th april, problem is i'm going to attend a friends wedding then do random activities such as snowboarding and so on and have no idea when i'd be coming back, or if i would be flying onwards to another country (been invited to Australia with some friends but theyre not sure of the dates yet either).

So getting down to it, would i have problems getting back into japan on a return-leg ticket WITHOUT having an exiting flight already purchased, if i explain the situation to the immigration officer?

If you don't have some sort of visa for work or spouse, the airline (not immigration) will probably not let you board with just a one-way ticket and no prescribed exit. Call your airline to confirm what you should do. Probably best thing is to get that onward ticket, so push your friends for dates.