Took it to Apple Genius Bar they said it needs a new logic board = £337 as the MacMini is less than 6 years old (5years 10months) I asked it they would fix it for free, under consumer law (worth a try!) Genius had a quick chat with store manager. They approved and replaced the logic board for free. I was expecting a bit of a fight, but there you go. Well done Apple :-)
Ribbon cable that connects the power button/prox sensor had broken just under the battery. Only thing i can think of is that the shock of dropping it, jarred the battery, and that put pressure and broke the ribbon cable. Anyway, took it to macFixer, and they sorted it out, as that job is a little beyond me.