This is what I found.
Bracken is also ericaceous, so it is one of the few viable substitutes for peat in growing plants, such as rhododendrons, camellias or blueberries in a container. I remember that when I was filming Real Gardens about seven years ago, I used to visit a gardener who used a composted bale of bracken every year to mulch his heathers. He said that it was all they needed to maintain the right acidity to thrive in his neutral soil.

The full bit is here:
https://www.theguardian.com/lifeandstyle/2005/oct/02/shopping.gardens#:~:text=Gardeners%20can%20use%20bracken%20as,a%20potash%2Drich%20soil%20conditioner.

As for when to cut it, someone will know.

What was not mentioned on GW was that Rhododendrons do not desperately need an acid soil, they just need a ready supply of manganese which is locked up in alkaline soils. The mulch of Rhododendron leaves provide the trace elements the plants need.
Home made compost is already acidic unless you add lime to it when making it.
Take care when cutting Bracken, the spores have been considered as causing cancer.
Only place I have ever seen Bracken compost on sale was at the NT place in Carding mill Valley.

Changing the soil Ph is virtually impossible. Yeh I know you can make temporary fixes now and then, but that's high maintenance and costly. The regulating factors are environmental.

Certain plants, such as rhododendron require a slightly acidic soil with a ph of 5.5 in order to take up all available nutrients the plant needs to thrive.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/acid-loving-plants.htm

Worth remembering that Rhododendrons are shallow rooting plants so a good depth of ericaceous soil would allow them to grow in what would appear to be unsuitable soil. I have seen them growing in limestone areas before, because their roots never reach that level.

Having read the comments from OB, I am in agreement that would have a concern with handling bracken. My main comment would be that as gardeners we are drawing in plants from all over the world and expecting them to grow in our gardens. It is my belief that I as much as I like certain plants, if the conditions in my garden are such that they will prove unsuitable then I would not choose to grow them.

Hello,

Honeysuckle, that's the article I was referring to, written by Monty Don. Given how strongly he's advocating bracken I was surprised not to find more references on the web. When you look up how to make ericaceous compost, however, there are loads of sites recommending coffee grounds and pine needles. Maybe it's just that most people don't have access to bracken.

Owdboggy and Bathgate, useful comments about leaves as mulch and PH. Thanks. I've read up about the spores, and will take precautions (like not harvesting it when spores are present).

Jimmytheone, I get your point, but Rhododendrons and Azaleas don't have any problems growing in my garden, I just had to cut back 3/4 of one because it's got way too big for the space it's in! I just want to make the best use of the resources I've got growing right here, which means making all my own compost rather than going out and buy bags of the stuff. The other reason to use bracken is that it's apparently high in potash, and I'm trying to encourage flowering principally.