This is an absolute disgrace.
Problems with the finish of your new home are covered for the first two years of the NHBC warranty. If these defects are reported to the developer in writing, which you have done, and the developer agrees that the defects are genuine i.e. they do not meet NHBC standards, which they have done. Then they are obliged to fix them. I would recommend the following:
Write to the developer to remind them of their obligations, stating when you expect a response by (2 weeks) and that you would like to be informed exactly when the remedial work will be carried out.
If this does not get results, you will need to take you complaint to the NHBC. One of their requirements is that defects are reported in writing, which is why it is so important to follow up conversations with site staff or customer service representatives with a letter. You have all this, so that shouldn't be a problem.
At the same time it is probably worth giving your trading standards office a ring, who may refer you to the Redrow's local trading standards office. One call from them might be enough to get the ball rolling.
You also have the option of taking them to the small claims court. They have admitted liability, so you have a good case. You have the evidence to prove this, so I would say the outcome would be positive. You can even do it online see:
http://www.courtservice.gov.uk/mcol/index.htm
Your claim would be for the amount it would cost you to get someone else to do the remedial work, plus your costs i.e. phone calls
I hope this helps, keep us updated.
Regards,
Tony