I think you've exhausted the path of trying to get Orange to remove the default. My wife had a similar problem, though with Barclays. We took it as far as we could, even to the financial ombudsman, without luck. Grudgingly I have to accept that Barclays were right, though like most people in this situation, there were mitigating circumstances. One further step to take is to write a letter to the credit agencies showing the default and ask them to append a note stating the reasons the default happened and that you paid it all off as soon as you could. It might not change a lender's final decision, but they DO have to take the note into account.
On the other hand, my wife was readily accepted for a joint mortgage application through our bank, so it turned out not to be as big a problem as we'd feared. Your own bank will keep an internal credit rating for you anyway and they might still see you as an excellent prospect for mortgage lending. Failing that, a broker will doubtless be able to turn something up. You might not get your pick of the very best rates, but I doubt there'll be a problem getting something competitive, assuming your credit score is otherwise ok.