Just to provide an update. This is now working well. Have done various field tests and all devices are reporting correctly. What is strange is that I haven’t changed anything except adding the virtual switches - this now allows me to spot if one or both devices are playing up. I wonder if this is/was an issue at the smartthings end as has been reported on before - either way, if this occurs again at least I now have visibility of the presence state of the mobile phones.
@Bry , no tablets are configured, just the phones.