Came up with this idea to generate a list of bookable slots from an easy interface. The time to generate the list seemed slow in GSheets but in the app it doesn’t appear to be too bad.
It currently does exactly what I want it to (apart from the American Date issue (not sure how to solve that as my brain is fried from the rest of this).
You can have 5,10,15,20 minute intervals as well, however they can break the arrayformula limit in GSheets so have taken it out for now.
In theory you can have any interval you want, however at the moment it calculates from 00:00 of “Today” so will break the following days if it isn’t a factor of 60, so you don’t get the option at the moment 
I am thinking that from a slot you can then “book” data into another table. From there I will (somehow) apply some booking logic back to the list to filter out the booked slots. Or show them as booked.
/edit I have a simple version of the booking done, just have to work out how to block out the future appointments if someone books a double (or more) session.