Right where do I begin!! (its so much easier explaining things with diagrams!!!)
I am going to assume you have bought a nice new aggressor from Planet which has come with its instruction "manual"!! (If it hasn't you won't understand a word I am saying!)
Syders basically come in two body shapes/sizes, Full or Compact. The "front expansion mount" is designed to fit on the compact body. The Aggressor has a Full body, however it has the same Receiver plug fixings (part 7, see diagram in manual) as a compact. This means the "front expansion mount" will fit nicely to the Aggressor (in place of part 2008) BUT the valve spring (part 10) is now too big. Therefore the valve sping (part 10) has to be replaced by the valve spring from a Compact Spyder. These springs are sold with the 32 Degrees and Madmann Spring kits (I think!), I do not know if anyone sells them in the UK.
Now if you have succesfully found a compact valve spring and done the above you will have a spyder with 2 holes to put the gas in. (the "front expansion mount" and the steel hose attached to the body) You will therefore have to remove the hose and the male to male adaptor (part 42) which is stuck in the body. Once part 42 is unscrewed, you will have a big hole that needs to be plugged, by ideally a short bolt/screw of the correct diameter.
Alternatively you can buy a front bottle mount that screws directly into the body (in place of part 42!) and will save you most of the hassle!!! Planet used to sell these (well they did in 1998/99!!)
I now have a headache and need to rest!!!!!!