Restricting different BBpress forums to different membership levels

I have two levels of paid access - level 1 & level 2. I also have a visitor level for non-members.

Using URL groups it is easy to stop the visitors from seeing the forums as the prefix forums is always present in the URL.

Likewise I can stop a level1 member from seeing the forum index (ie it's archive) page that is for level2 members only.

However, I cannot seem to have forum topics that are visible to the level2 members and protected from level1 members.

How can I can I achieve this?

Many thanks