Away from my books at moment.
Was there a Mystery (and if so, where?) that allows you to add extra enchantment space in a Talisman, or similar?
ie in the case of a talisman composed of multiple items, you don't open all the components, only the most Vis consumong.. How do you later add more Vis in order to open more Enchantment spaces?
No Mystery needed.
Once an item has become your Talisman you can easily increase the available space for Enchantments.
You just spend a season and the number of pawns of vis you want to increase the space by. Max spaces is highest Technique + highest Form.
ErikT beat me to it. Talisman are unique and use their own special rules for enchanting which includes special capacity calculations.
For things other than Talisman, I don't remember any option for increasing it after it has been opened.
Not after you have finished preparing a non-talisman for enchantment, no.
There are a couple of mysteries that can help you increase the amount of Enchantment space in other items, but they need to be used before instilling any effects:
The Verditius Mystery Item Attunement (HoH:MC), and the Hermetic Mystery Philosophical Alchemy (TM:TR)
You might have recalled the Mystery Virtue Great Talisman (TMRE p.89f). You don't need it to add extra enchantment space to talismans, but to craft additional components into them.
The Verditius Minor Mystery Reforging Enchanted Items allows items to have their enchantment space increased after the fact.