In Sunbitterns there is no difference between male and female ( at least not visiable ).
Rings can indeed used to tell the sexes apart. At Walsrode we used to ring the birds afther they had been sexed the following way, males got a ring on the right leg, females on the left leg. Also colored rings can be used, for example blue rings for males, red rings for females.
Don't know however if Twycross and/or Chester use one of these systems..