6 weeks is I'm afraid allowable for surface mail - I get many US journals and though many make it within 4 weeks, 6 or sometimes 7 is not unknown.
The Sept 11th would I suspect have made a major difference as US airspace closed to non US planes for many days - thus mail must have been stacked up and surface mail is always lowest priority. There ware also labour disputes at Liverpool sorting office around that time - these too may have delayed matters as I suspect all 2nd class mail ex Island probably lands through there