Hi there Sir/Madam,
I am by no mean an expert, but can offer an opinion.
The way most people do it is to gain their PPL, and then satisfy the prerequsite requirements for the CPL course, by completing 150 total time including 95 hours pilot in command, then crack on with the ATPL writtens, 14 subjects usually spread out over the period of 6 - 12 months. However, more and more people seem to be getting their PPL's and then going straight into the ATPL written study, then doing their hour building requirements, night and all that.
I seem to recall under the old system that you had to have a PPL and 100 hours or something like that to start the ATPL writtens, however, under JAR I do believe that rule no longer stands, someone please confirm. As long as you have an ICAO PPL you can crack on with the ATPL writtens.
I think with the above in mind, it would be a nice idea to get the ATPL writtens out of the way on completion of your PPL, and then just go for gold with the flying, at least then you will have little or no gaps between your hour building/flying training. I currently hold around 135 hours TT, but have left myself around 25 hours PIC to do once I have finished the ATPL ground school, then have planned to go staright into the CPL.
As for the actual requirements for the issue of your ATPL I am not sure. All I know that it is 1500 hours TT, but am not sure of the specifics, i.e. night, PIC, cross country IFR etc.
Hope the above helps
.