Hi Ian, I was thinking the same just wanted a second opinion.
I had already done the fuel filter as basic checks and with bubbles from all four injectors I kinda ruled out combustion gasses as I wouldn't expect all four to be leaking exactly the same.
So I went back to the basics and found a small crack in the fuel supply pipe between the filter and pump. I can now confirm there should be no bubbles from the leak offs.

Chris