I don't have a microwave either. I put it in a pan with cold water & heat it on cooker slowly, stirring with a wooden spoon. I never let it get anywhere near boiling, but it does thicken up nicely... this has been part of my routine from the start. Once I was away for the weekend & tried to make it in mum-in-laws micro, it kind of went everywhere, like the quatermass or something... cooker-top is way easier, but don't let it boil!
Y'know, as I was writing the above I had a fear deep down that it wasn't allowed... me and my big mouth! Thanks KD, too late for today, but will try tomorrow. I had been told it was fine as long as you didn't boil things, and thought that if I stopped at heating it to warm I would be OK. At least you have told me a solution - would be lost without my porridge!