Hi Callum,
That extension might be OK for now, very whippy, but any higher than that, then I would start saving the extra for an SLX carbon fibre pole.
I found the clx great when I first started wfp, then I went to SLX and the extra rigidity made the job less tiring on the muscles. The pole is not as whippy, especially over connie roofs etc.
Also, the SLX range gives you more add on sections when you may require them. A very versatile pole. Great all-rounder.