Trick Play Legal?

[coyotes21]

I play for a flag football team in the UK, its too late to complain now but we lost the national finals on Sunday to a "trick" play in overtime, can anyone comment upon wether it is legal or not.

First play of overtime offense comes oton the field 10 men go into the huddle play is called offence then brakes from huddle, an 11th man then (slyly)steps onto the field as a reciever with no corner on him and cactches a TD!!!!! Does he need to report onto the field or check in with thye huddle.

Thanks