The problem has returned - NWOLB wouldn't run in either Firefox or Chrome - but was quite happy in Safari.
It seems that NW have again been setting some cookies that can subsequently cause a 'security exception' when you try to start a banking session in Chrome or Firefox.
Easy solution is to delete all NatWest cookies from your browser and hey presto it works again - only irritation is that it loses your saved Customer Number - so make sure you know this before clearing the cookies.
Alternatively, start a 'private' or 'incognito' window for your banking session.