Now that I have finally come up with a relativly easy method of butt joing bottles, I sat down this weekend and made another two butt to butt joined sections. I have that it is extremely important to make sure that the holes in the bottom of the bottles are perfectly central, otherwsie the ridges on either bottle will not sit together properly. This will in turn mean that the actual centre of the bottles are not touching and thus your little piece of threaded rod, will not be long enough.
Now that I have 3 completed sections, I can start to construct the bottom half of our new rocket, which will be a booster cluster comprised of 3xrockets. For demonstration purposes I have held the 3 sections together with elastic bands.
You can see I have placed the staging mechanism on top of the boosters. There will probably be another section inetween them, which contains the electronics and parachutes for the booster section.