Have noticed our BES express db is in the region of 4gb, we currently have 30 users on this. Yet our Full BES 5 server has significantly more users and yet is only 400mb

Any thoughts on this - as kind of think there is a problem?

I'm guessing your SQL purge jobs aren't running. Check out: KB14017-How to Run Microsoft SQL Server maintenance using Microsoft SQL Server 2005 Express Edition

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Sorry should have been clearer ,the express is the BES side of things
Running BesXpress

SQL side we are running full 2005 SQL server.

Not sure if this is related but have also been experiencing large transaction logs, see previous post
Transaction Logs Getting Full

At the moment I am having to manually kill the RemoveOldMSAddresses process from SQL each day, in order that I can log into the BES and add any users.