Thank you for your response.
I spent considerable time yesterday using the automated facility, trying to resolve the problem without success. In fact I found this process useless!
I eventually managed to speak to a human being, who could not deal with the issue and transferred me to a colleague. After going through the rigmarole of security checks again, it turned out that she could not help me either, and after some time transferred me to yet another colleague.
After having to go through security checks yet again for the third time, she told me that she could help me. After putting me on hold for a while, and after one hour and six minutes on the phone, she told me that I was able to resolve the issue myself by dialling 1470 followed by #, followed by 227#.
At last I thought, I thanked her for her help and rang off. I then did what she had told me, but absolutely nothing changed. My number is still being withheld!
I find it absolutely extraordinary that such a simple issue cannot be resolved.
Could you please arranged for someone with a bit of technical ability to resolve the issue and confirm that it has been done.