Hi Pauline, I have a friend who has to have her tea so strong one could stand the teaspoon up on its own in it. She too uses only Yorkshire Tea as she believes it has more of what she calls 'tannings' in it. She gets a really strong cuppa by pouring boiling water onto the teabag, giving it a quick stir, leave it for 3 minutes, then stirs it slowly to a count of 20.
It certainly works for her, and its how I now have to make her a cuppa without her politely wincing at how weak it is.
If we are out, she never orders tea because no-one can ever make it how she likes it.