Thanks for the advice, pussygalore and Maria22. The pasta was cooked fine, following what basically sounds much like pussygalore's approach, although I wonder whether Exante's recommended 150ml water is maybe too much. The flavour wasn't too bad, though, so I wouldn't rule out eating it every now and then to relieve the boredom. If I buy another sachet or two, I might try Maria's method next time.
I'm hoping the porridge isn't too sweet because I'm getting a bit fed up with sweet things (I have a soup, bar and shake every day, so two-thirds of my meals are sweet). But I'm saving my one-and-only porridge sachet for next week, when I am sure I'll be ready for a change!