Ok - Here's event log ID, <Value> in my case is 209 and I am getting the impression this is an inheritted default from an upgrade.
Event ID: 1164
Source: MSExchangeIS Mailbox Store
Component: Microsoft Exchange Information Store
Message: No database session was available to satisfy a request on database "<name>".
The number of sessions configured via the Active Directory attribute msExchESEParamMaxSessions on the storage group maybe set too low for the level of activity on this storage group.
The current value for this parameter is <value> 209.
I found this link on our lovely EE site but my adsiedit in MMC doesn't sound at all like his and I cannot drill down to 'config' as he suggests. I need to modify this value , does anyone lnow how???