Take a look at two settings on your router.
1. DHCP lease time. Set it to at least 24 hours.
2. Under 2.4 wireless turn off Group Rekey Interval.
By it repeating at the same time daily I would still suspect it to be a router issue. It may be that the devices losing connection are in weak signal zones. They may not be getting the WPA2 rekeys. Another issue but less likely is that something in the home or neighborhood is cycling and causing interference at 3:00 am. Changing the wifi channel on your router might cure that.
I don´t know about the UK, but here in Germany every day the internet connection is cutted for a moment from provider. It´s a rule here. At which time in day I can choose in router (at me it is set to 2am).
So every day at 2am all my devices loose internet connection. Most of them get it back within seconds, but some need more time. You wrote about 1,5 hours, so much it is not at me, I would say maximum 10 minutes.
Maybe something like this?