Thank you both for your replies.
The database is MSSQL and unfortunately is in simple mode so I can't take the approach of restoring and then replaying transaction logs to a point in time before the add-on installation.
Reagan, yes, I think the stage you mention must be where I took the post upgrade backup. I do have a backup of both the database and SUM directory at the point before the downtime commenced, it's after the downtime that I don't have a backup of the SUM directory.
With this in mind I'm wondering if restoring the database and SUM to the point before downtime is the better approach? I'll have to go through downtime processing again as I'm restoring to a point further back however doing so will ensure that postprocessing runs - skipping which is my concern with restoring to the post upgrade backup as I don't know how critical the tasks it runs are.
With resoring to a backup before the downtime commenced, can you continue from that point or do you actually need to reset the upgrade and then start again? To my thinking the DB will contain all the shadow tables and the SUM directory will be correct but the shadow instance service won't actually exist on the server.....
Thanks again.