You must make a return every year by the 31 Oct following the year in which the gain arose, so for a gain in 2006 it must be filed by 31 Oct 07.
As for making payments the rules differ slightly:
The rules are:
For a gain made between 1 Jan and 30 Sept the tax is due to be paid before 31 Oct in the same year, just a payment not the return
For a gain made between 1 Oct and 31 Dec the tax is due to be paid before 31 Jan in the following year, again just a payment not a return.