Chi chi, don't worry, that is exactly what happened to me when I tried to go for the long cycle, it is quite a normal response.
I am a patient of Professor Studd too and like you, I am severely intolerant to all types of synthetic progesterone and have to limit my use of Utrogestan. I use 2 pumps of Oestrogel everyday and can only go about 5-6 weeks before I shed naturally (aka breakthrough bleeding) but it is just like a very long period that lasts continuously for about 2 weeks and then it stops completely and I get no bleeding at all - it is not random or sparodic, it is one long period.
In order to avoid this happening again, I now suffer a few days of Utrogestan which, luckily, I don't need much of to get womb clearance (been proven by a scan) and that has done the trick. It is the same volume of blood loss but it all comes out over a period of about 4 days as opposed to the dreadful 2 weeks.
Long cycle is completely out of the question for me because I shed naturally once my lining gets to a certain point and I don't want uncontrolled bleeding.
If you are worried about the Mirena, is it worth having one last try to the Utrogestan? Try using 100mg for a few days vaginally and see how you feel and what sort of bleed you have. I have found that if I use it in small doses, it still works and I can tolerate it.