Mira Sport - Low flow (only scorching hot water available)?

I've got a Mira sport shower, probably bought around 2004-5 so not that old. Lately it's started to show "low flow" when on anything other than burning hot temperatures so I either have to turn the power down to medium (which is a trickle) or keep turning the temperature from hot to cold (which then trips the low flow again)

Any ideas?