Frozen after my rode so have no real inclination to try and open the casing right now to assess the damage.

Mid ride nearing the top of the first climb the up shifter when pushed down just kept rotating without engaging the gears.

Is the shifter likley to be toast?

Is it the twist grip (you refer to rotating) or trigger? Both are actually fairly straightforward to take apart.

How cold was it? Could it just be that the cables / mech have frozen up?

Trigger shifter, -2 to -3

The down shift trigger was fine just the up shift when pushed down/in kept going down without engaging of the gears. Was thinking that there might be an issue with the pawls.

The lever position is adjustable – the black bolt underneath I think – maybe that’s just come a bit loose and needs tightening up?

Thanks Dberry I’ll give that a go later this evening