The 2mtrs section swr is 1.3:1, whereas the two slots are 1.2:1. The shorter 492mm arm should not be in front of the 23cms slot, and should sit just below the slot and not in front. Place at 90 degrees to slot if necessary.
The 57mm matching may need to be lowered a little for a better match, due to the larger diam 35mm pole.The inner connects to the large pole and the braid/screen connects to the 15mm short pole.
Pipe and joints available mail order from /www.bes.co.uk/
For construction details of the slots see www.g8ajn.tv/23cms.htm
Plastic drainpipe can be used to protect the pole from weather, but I found a wrap of black damp course DPC sheet had less impact on the swr. To test a pipe for suitability as a cover place a small piece of the pipe in a microwave oven with a cup of water(as a load) and run for 1 min on high power. If plastic hot or even warm on removal, it is not suitable. Rainwater plastic downpipe has proved very successful.