It is a place you can visit, but you need to make an appointment. Contact Isobel (her email/phone number should be on the website) and if she has enough time she will pull out items specific to your interests. I didn't end up making it there myself (I was unfortunately with 3 others who didn't want to go) but she will also make up PDF files to send to you, which I intend to do.
We did wander through Berkamsted and it is a great little town.