Maybe I was dwelling too much as to what could have happened.
The problem was the other brake pipe. I asked the mechanic "surely you checked the pipe when you replaced the other one?. He advised, he had checked it, but that the pipes are made of steel so they are always rusty and covered in muck, so its difficult to detect any problems.
The lesson in all this is, make sure your seating position allows your foot to reach deep into the footwell, so you're able to operate the dual system brake.
Thanks to all those that helped.