Welcome to the Club, Mark.
You have a very nice car and you have taken the trouble to have the ceramic paint protection system, which does last quite a long time.
The dealer is quite right is saying that all you need to do is use plain water.
I had my [then] 6 years old s type in Indigo Blue given a paint protection coating 3 years ago, and when washing it I use the three bucket system. A gentle wash using a microfibre cloth, and then a second bucket just to make sure you haven't missed anything. The third bucket is a good rinse.
I always use microfibre cloths, or cleaning "sponges". I then dry the car using a towel from Halfords specifically made for the purpose, I was advised by a professional detailer not to use a chamois leather.
Lambs wool is said to be good, although I have never used it.
My car sits on the driveway and is our daily ride.
I hope this has been useful. We do make good cars in the West Midlands!
Regards
Peter,
Taken after rain - my s type, the reflection on the bonnet, and the third is a close up of the bonnet after rain.