The solenoid fits nicely inside the flap ferring (http://www.ebay.co.uk/itm/141673673315?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT). I drilled two holes on the outside of the flap, but on the inside ferring, to allow the solenoid to operate freely. It’s the type that’s usually retracted, so if there’s a power cut the cats can get out. There’s a few heat issues, which I’m hoping a few vent holes in the cat flap (http://www.amazon.co.uk/SureflapSureflap-SureFlap-Microchip-Pet-Door/dp/B009NH6NR0) will fix.
I’ll post better pics when I drill the vent holes tomorrow.