Thank you all for your comments. I think Grant has put me on the right track though because it's not just one report query that's slow - it's everything. And if I run a stored procedure after the database has been updated it grinds through - then when I run it again it zips off in no time.

I believe the database is being set offline before the logs are applied and then online again afterwards. I might be wrong. The database is also set to read-only. And both those actions will clear the plan cache according to Microsoft.

I don't think either action is necessary. And I have a friendly dba to talk to about it!

Again - thanks for your comments.