Managed to solve this one about 10-15 minutes after posting this (!) - downloaded SQL Management Studio, connected to the appliance, browser for the table USERENTITY then modified the T-SQL found here to reflect the SE200's SQL structure:
/****** Reset 'admin' user password on SE200 *****/
Update [ReadiManager].[dbo].[userentity] Set Password='qwRGwD79BAw=' where EntityID=1;
I hope this is useful to other members of the community that experience a similar problem!