Can I have extra free time to shower, dress, and generally hang about enjoying the warmth and ambience of the dungeon for nothing?

No. 

The Dungeon Keeper will expect to have access to the facilities at the end of your hire to prepare them for his or others use. 

If you wish to extend your hire you will need to have arranged this with the Dungeon Keeper at the beginning of the hire at the latest. Circumstances might make it not possible to extend your hire.

If you do run over you will be charged for the extra time. Extra time is charged in minimum of 30 minute units.

There is usually at least two hours gap between hires so you will not be expected to rush off. However you will be expected to be finished and ready to leave at the end of the time you have paid for. Your time is for you to use as you wish... The Dungeon Keepers time is his time and he is not really interested in giving it away for free.

However he is a friendly chap and, if you have not trashed his pride and joy, he will probably enjoy chatting with you to ensure everything was as you wished and that you had a jolly good time. So you wont be rushed out the door and if you need to wait for your cab to arrive that is fine.