You can see guidance on the meaning of dormant for Corporation Tax here:
Corporation Tax: trading and non-trading
If you meet the criteria, which you appear to, you do not need to file a company tax return.
As there is no direct link between Companies House and our records, we will need to manually update our records to prevent the issue of notices to file. Please contact our Corporation Tax team and we will, in most situations, be able to update our records from a telephone call.
Corporation Tax: general enquiries
The not authorised message from the Government Gateway account does not relate to the fact the company is dormant, we will only need to look at this if we are not able to note our records as dormant.