Take the rubber handgrip off using heat, fit an Unger alu N95 section onto the base of the Harriss.
Also an Unger top No1 telepole standard section fits inside the Harris No1. Use one of the intermal springs from most Unger tools and drill two new locator holes in it. Or wrap some pvc tape as a stop on the Unger section and insert into the Unger No1 until it wedges- not a telscopic but just that bit more reach.
Personally if you need that extra height,on a regular basis- get a proper C/F Map M16 from cheshire fishing, or Stapely water gardens etc.