Yes the banks will have to honour them, retailers dont as from the 1st June. So in effect the note is useless
http://www.simplybusiness.co.uk/knowledge/news/2010/03/2010-03-09-retailers-warned-over-20-pound-note-withdrawal/
As of tomorrow the old style notes will no longer be legal tender, only found out by accident so off th the bank I go
http://www.bankofengland.co.uk/banknotes/current/index.htm