@ Airburst
Sorry, I didn't pick that up. That's interesting ! It emphasises how difficult it is to pin down exactly what the situation is.

So, if I have 27 Shimano gears, then I can use SRAM Attack Gripshift at the back for both MTB and road, but in all cases, but the front depends if it is MTB or road - if it is road, then I have to use a Gripshift designed for road gear (as they are not indexed).

Hopefully, that settles the situation for Shimano gears - which I will have on my next bike.

thanks to everyone for your help