@Zia As a Chester member, I have contacted reciprocal zoos before visiting via Facebook messaging and have found that they are still honouring the scheme. They might just ask for your details and have you collect a ticket from the service kiosks before entering.
Thanks @littleRedPanda - it does seem to vary. We had to buy a ticket for Paignton if we wanted to arrive before 1pm but for Twycross we just emailed and were added to a list. I believe Marwell were not allowing reciprocal member visits at one point. I have messaged them on Facebook but just wondered if anyone had already queried it.