If there has only been a couple of wrong codes entered then to get rid of error message, leave switched ON UNDISTURBED for an hour or so.
I don`t think anyone on these free Help Boards can calculate 8 digit Serial Numbers .
Only ones beginning with M ****** ( or possibly L , or C or 7 digit numbers)
Useful info here
www.tinyurl.com/4ydhop
RegWorts
July 2008