Very easy to do.
Step 1: Cut the threaded end of the Harris pole off using a hacksaw just below the rivet and throw away.
Step 2: You now need to replace the threaded end with one that will accept the angle adaptor for your brush. You can use this adaptor
http://www.wfpshop.co.uk/ or buy a household broom from Tesco and cut 3 or 4 inches off the threaded end of the pole. This will slip into the end of the Harris pole, you can either araldite this in place or hold in place with one or two small self tapping screws.
Step 3: Drill a 12mm hole into the bottom section of the pole. (to make it easier drill 6mm first then open the hole out with a 10mm and finally a 12mm drill bit)
Step 4: Approximately 6 inches or so from the threaded end of the pole drill a 10mm hole on one side only (Only drill into the hollow section of the pole and not through and out the other side). Use a file or coarse sand paper to remove burrs and sharp edges.
So now you have a Harris pole with a threaded end to accept your angle adaptor and brush, a 10mm hole 6 inches down where your pole hose will exit to be attached to your brush and a 12mm hole in the bottom. Now thread your pole hose into the 10mm hole and out the bottom end of the pole, make sure you use enough pole hose, I would recommend about 8 mtrs plus.
Job done, it sounds long winded when written down like this but it is really easy and can be done in 15 mins.