when does my visa actually start?

I've recently applied for (and received) a working visa for Japan, giving all the usual intinerary and other info needed. I stated I was to fly on 12th March. When my passport came back with the visa the date of issuse was 2nd Feb. Does this mean I can only stay in Japan til 1st Feb or will my visa start when I enter Japan, and it means I have a year to put my visa to use?

The period of validity of the visa on your passport signifies the period in which the visa can be used at the time of entry into Japan. The permitted period of stay in Japan will be given by the immigration in Japan airport.

Your "JAPAN VISA" is one thing, you get a "LANDING PERMISSION" when you arrive in Japan at the airport, you can stay in the validity of your "LANDING PERMISSION".

When people talk about Visas here they are in reality talking about the "landing permission" then the actual visa.
My visa has a FEB issue date, but my landing permission has a APRIL 20xx - APRIL 20xx validity.

You have three months from the issue date (2 Feb) to enter Japan. Once you enter your 1 year stay will start from the date you landed. If you don't enter Japan by 3 months from the 2nd of Feb you visa would expire and you would have to start the application process again. Since you plan to arrive on March 12, this is not an issue. Your "period of stay" will be until March next year.