So it's all installed and working exactly the way I envisioned. Press to engage Sport mode, press to re-engage Econ mode and so on. The existing switch didn't like the piggy-back arrangement and refused to work when reconnected, so after checking it still worked without the new indicator-stalk switch, I refitted it and left it disconnected. At some point (most likely next winter now!) I'll fit an accessible and discreet Snow mode momentary switch to the right of the gear lever as the new switch isn't capable of dealing with that.
If you're doing this mod, be aware that you need to
reverse the polarity on the trigger supply to the relay coil from that shown on the diagram above, as the alternating outputs need to be 12V negative. The supplier confirmed that there is a brief connection made between 56 and 30 when the switch (S) is released. I had to query why I kept blowing supply fuses and it transpired this was the reason. The relay actually has a printed circuit diagram on the top which does (not very clearly) show this, unlike the diagram above. So:-
30 negative (to chassis)
S positive (from switch on indicator stalk)
56 negative (to chassis)
56a (to Yellow/Orange wire (pin 4, Econ mode) on existing tri-switch connector)
56b (to Yellow/Black wire (pin 5, Sport mode) on existing tri-switch connector)
The switch on the stalk has two separate wires (brown and black) with a small 2-pin connector which needs to be cut off. Either of the two wires should be connected to an existing ignition-controlled 12V+ feed, and the other runs to the S connector on the relay as above. The indicator stalk is otherwise identical to the old one and is simply connected to the multiplug and fitted back onto the steering column bracket with two screws.
The one other operational difference with the new switch is that if you turn the ignition off whilst in Sport mode, it remains in Sport mode when you next turn the ignition on. This is due to the relay having mechanical switching. As it's relatively uncommon for me to use Sport mode, I didn't consider this an issue and having used it stop-start today, it really isn't.
My thanks to everyone who helped with this pleasing mod. RoverP480's link to the latching relay worked out perfectly as it's the only type whose function does what needs to be done. I got it for £22 next-day from
this eBay seller at no delivery charge which was outstanding.
Dougie.