as there are no mantids native to uk, i have never had to/been able to do this. however i have read about people leaving female moths outside over night to attract males. this is usually done by leaving the females in a net cage. i think sometimes people use very coarse netting to allow the moths to mate through the netting. however the way mantids mate i doubt this would be possible so the best you could hope for would be to keep the female in a net container and then keep an eye on her container in case some males drop by. then catch the males yourself and try mating later. maybe even put a light near her? i dont know if this would help attract mantids but i would assume so.