Emma
From comments on this and other fourms, it would appear that the DWP are currently under severe strain due to the IB/IS/SDA transfer, which has only exacerbated by the Cristmas period, so it is not entirely surprising that they have not replied to you.
That said, you should have had acknowledgement of your request within these timescales. Normally I would suggest the claimant phones the DWP, but obviously this is precluded in your case, I can only suggest that your write to them asking for an update.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems