Hi Neil,
Good question, but can I suggest you first reading the previous post ... before asking your questions ()

Now as far as I am concern, I visited the K 3 KEROMAN sub-base in Lorient (France) last year and can tell you the following tips.
You can't visit the location at your own, you are obliged to do it via guided tours. These are organised some days/some hours in the week and w-e.and commented only in French (I guest). Price (2007) : adult € 6. Off the high-season (off July/August) it is only possible on Saturday and Sunday (only 2 runs p.m.)
For more info, apply (or surf) for the official organisation : L'ORIENT TOURISME Maison de la Mer. Quai de Rohan. 56100 Lorient. Tel . : + (..) 02.97.21.07.84. E-mail : accueil@lorient-tourisme.fr This is certainly a visit worth !

There is also the possibility to visit (independently) the "Davis Tower" on the same site, it was the training-simulator for crews for escaping an u-boot in case of emergency. This small museum (located next to the Capitainerie du port) is run by volunteers and there is no connection with the above mentioned L'ORIENT TOURISME? !! This simulator, and also the entire base, were used by the French Navy from 1946 until 1995 !!

St. NAZAIRE : I have been there a few years ago.
You can walk around the shelters, and there is also a (French) sub-marine to visit + museum + a sheltered-lock !!
Also ask info via : www.mairie-saintnazaire.fr - click on the British-flag above/right and then on the titel : "submarine base".

Fair winds and pretty hollidays !
Freddy.
Belgium.