Out of curiousity, could you use on of these inline heaters to provide
a tiny bit more oomph to a shower that runs from an existing combi hot
feed? I'm presuming this wouldn't work as the inline would restrict the
water flow too much but I'd be interested to find out.

My guess is that the inline wouldn't actually produce significant pressure
difference and could be left in line and would work as you indicate. I have
no data to back this up, though.

However, even a diddy 24kW combi should have enough for all but the most
powerful showers, such as panel types.

Christian.