When I was employed at a company with an ESPP (purchase instead of ownership; I'm assuming yours is given as a bonus in your compensation package while ours was purchased at a discount), I sold most of the stock I owned as soon as I got it for the "eggs in one basket" reason. I still think it was a good choice, but knowing the market results I would have only changed my timing :).
In the US, when you hold an investment longer, the earnings are taxed at a lower rate (long term gains vs. short term gains). Do you know if that's the case where you are? You will pay the other taxes at some point no matter what, correct? I personally would sell when I could with the lowest tax implications (in the US, that would be a low earning year witha small bonus, without other gains like a home sale, or no other large taxable stock income). Also be cognizant of the fees to sell. My company's broker of choice charged $75 to sell plus a $22 handling fee (I was able to move the stock free from them to my more reasonable broker, $7 to sell). If that's the case for you as well, minimize the number of times you sell, or move it if you can.