Im in the same position, so i emailed Angard Staffing and this was there automated reply..
'Frequently asked questions
"Hired and agreed start date" on
http://www.royalmail.com" onclick="window.open(this.href);return false;
If you have recently logged into your application on the Royal Mail website
and your status is showing as "Hired and agreed start date", this means you
have passed full Security Clearance and are able to work at Royal Mail
through Angard, and will be contacted in due course as and when shifts
become available. This is an automated status, and it doesn't automatically
mean you have a start date agreed. We contact candidates on a shift by
shift basis and this status will enable us to include you in our searches
for available candidates'
Hope this helps :)