Poor Ferdi! Some cats are just anxious by nature, but there are medications that can make a big difference and help long-term.
I would definitely keep the Feliway diffuser plugged in and full at all times. It covers about 900 square feet of space, so if your home is larger than that, you should get a second one.
Is there a reason they go outside during the day? Or is Ferdi even more anxious if kept in all the time?
I would definitely ask your regular veterinarian about starting Ferdi on fluoxetine, which is generic Prozac. I have used it in many cats with anxiety and it does help. It often makes them sleepy, so I recommend giving the dose at night, when he is most likely to be sleeping (or you are sleeping and wouldn't notice sedation in his behavior). It is only given once a day, which makes medicating him a little easier.
It is also important to try and stick with a routine. When visitors come, just let Ferdi hide out under the bed. That is where he feels the safest and there really isn't a reason to force him to interact when he doesn't. It just makes him more anxious the next time. Let him have his safe spots where he isn't disturbed. If he does come out, then reward him with a special treat and praise.
We let them out as we are in a fairly safe area and they have an 80ft by 40ft garden with high fences they use as pathways. They have free access and egress in the day and litter trays available at all times. We will get another feliway as the bungalow is fairly large. We do take them to a local cattery when we are away and they are used to that although Ferdi stays mostly in his travelling basket when the girls are working in there. They say he does allow them to brush him tho I expect thats not happily. I am taking them there while my daughter and family are visiting so he will not be afraid in his own home. Thank you for yourhelp. Wendy