It's very odd - the water backs up until about 5 minutes into the shower, when it's as if someone has "pulled the plug" and it drains away, and tends to run freely from then.
Obviously we've de-haired the drain, put unblocker/bleach down, sometimes helps, sometimes doesn't, but if it was blocked it wouldn't ever clear, would it? Same as if the water pressure was just too high. So what can be causing this?