I ended up deletiing all the Best Bets from the DB, as it's just a dev environment so no great shakes to lose them all. This worked fine for me, but obviously hard deleting stuff out of the CMS DB comes with risk attached. Posting the script here is case it's useful to someone, but it's quite clearly unsafe so only intended for use on dev databases; back up DB before use, check the items it will be deleting before running it, cross fingers and hope for the best (bets)

DECLARE @ids table (id int IDENTITY, val int not null)
	INSERT INTO @ids
	SELECT pkId FROM tblBigTable 
	WHERE ItemType LIKE '%BestBet%'


DECLARE @number int
DECLARE @bestBetId int
DECLARE @count int
SELECT @count = COUNT(*) FROM @ids i

PRINT '' + CAST(@count AS nvarchar) + ' items to process'

SET @number=1

WHILE @number <= @count BEGIN

	PRINT 'Processing item ' + 
		CAST(@number AS nvarchar) + ' of ' + 
		CAST(@count AS nvarchar)

	SELECT @bestBetId = val FROM @ids i WHERE i.id=@number

	DELETE FROM tblBigTableReference WHERE pkId=@bestBetId
	DELETE FROM tblBigTableReference WHERE RefIdValue=@bestBetId
	DELETE FROM tblBigTable WHERE pkId=@bestBetId
	DELETE FROM tblBigTableIdentity WHERE pkId=@bestBetId

	SET @number = @number + 1

END

Hi,

This bug is fixed in Find Commerce 9.6.1 and will be released next week

/Q