So I was able to play today, and a lovely day it was too.
Therefore I was able to progress the intermittent wiper install.
Firstly, remove the lower cowl from the steering column (7 screws underneath on mine) and then drop it to it's lowest position.
Press in the clips top and bottom of the existing wiper stalk and draw it out sideways; the attached multipin plug will come with it. The unplug the stalk from the plug; they just pull apart.
The wire we are after is the orange/purple.
Cut this. I then put a male and female bullet crimp on to the cut ends, so that if it all went awry, I could just plug them together and rejoin the cut.
I was lucky enough to have a connector, with some short wiring, that fits directly onto the additional connector on the replacement stalk, and so was able to add a male and female bullets to the 2 relevant wires; if you don't have this connector, then you will need to cut off the plug and put the bullet connectors directly on the wires as they come out of the stalk. The wires needed (there are 5 in total) are black/white and black/yellow. Since we are just wiring in a potentiometer, it doesn't matter which way round it is connected. Ignore the black box connected to the other wires; that is to connect the switches on the end of the stalk to my stereo (which kinda worked; think I have an occasional short somewhere

)
Plug the bullet connectors together.
Then put the multipin plug back onto the rear of the new stalk (as in 'reassembly is the reverse'). Slide the stalk back into the metal frame on the steering column until it clicks home and tidy up the wiring; it fits easily in the space available

OK - now onto the relay. The one we are after is the black one right in between the green, red and yellow:
Simply, pull it out and replace with our new one. Hold it this way in order to fit.
Having done this, happy to report it all works as expected; existing functions are unaffected (wash/wipe, slow and fast) except for the intermittent setting where the interval is controlled by the setting of the wheel on the stalk - a complete result. Item 8 ticked off the list

It is really simple to do; sounds much more complex than it actually is. All in all, about 30 minutes to achieve. If you want to try this yourself, normal caveats apply...
There are 10 kinds of people in the world; those that understand binary.....