GLOW WORM ULTRACOM 30 HXI (F5 - OVERHEAT RE-SET?)?

THIS IS A 2008 MODEL.
I have had air in the system & the boiler now displays F5 & will not start.
I believe there is an overheat re-set switch but can't locate this. The manual shows a thermistor on each of the flow & returns (both are present on this boiler) and the overheat switch is shown between the flow thermistor & the vent valve (but there is no sign of a switch on this boiler)
Could someone help me with locating the switch or is there another method of re-setting for this model?