I've been asked by email:
I would be very happy to discover during one or two days some examples of coastal batteries and command posts pillboxes (gin drinker's line ) and perhaps shing mun redoubt.
Is it possible to meet somebody in HK who accepts to show me some examples of theses fortresses I could show him something similar if he comes to France.
Please give me if you can find me a specialist who is motorized with a car (name and phone number and available, with a car ?) and a good precise map of fortresses.
DIY / Self-guided
The easiest way to see coastal batteries is to visit the Museum of Coastal Defence. It is built in an old coastal battery. There is a walking trail around several of the gun emplacements, and there are several interesting exhibitions inside the main building.
You could also walk the Wong Nai Chung Gap Trail, described at the bottom of this page. The site commands the island, and was the scene of fierce fighting in 1941. It's away from the coast, but there are remains of an Anti-aircraft battery and pillboxes to see.
The book "War Relics in the Green", ISBN 962 993 295 4, introduces a variety of old fortifications in Hong Kong, together with maps and directions on how to visit them. I can't find it to order online though, and there's no mention of it in the government online bookstore, so it may be out of print.
The only company I know of that runs guided walks is "Walk Hong Kong". They offer a range of WW2 Battlefield walks, led by Martin Heyes.
Maps of fortifications
Gwulo is a good source to pinpoint the location of old fortifications. See one of the pages under "Historic Military Buildings in Hong Kong". Once you've identified an interesting location, use the government's GeoInfo Map to get a better idea of what's there today.
Can readers recommend any other options?