I solved my login problems.
The issue for me was that i was tring to access the database with a database user account which didn't have full admin control. Reverting to the DBO (database owner) user account solved all the probelms. This circumstance should hopefully be rare for other users but may be useful to note.
(My ISP (fasthosts UK) was hacked and in response reset passwords for all database users as a defensive measure. When showing database users in control panel they don't list DBO in the users list making me think they had killed the user account - so I recreated a variant but it didn't have full admin control - but did reserect the site so users could see it after 24 hrs down.)
Andrew