Monarch in Hertfordshire, U.K.

[J.B.Murray (John Murray)]

I have a report of a Monarch butterfly seen in a garden at High Barnet at 15h 
yesterday afternoon (August 12th).  Since this is only the 2nd ever record for
Hertfordshire, and the observer was not one of our usual recorders, I am asking
him for a full description of the butterfly and the sighting.  

If correctly identified, I am intrigued to know if this is a genuine immigrant,
or an escape from a butterfly house or individual release.  Does anyone have
details of any U.K. sightings this year?  Immigrant moths are starting to appear
and I saw the first Red Admiral yesterday, so that would tend to suggest an
immigrant is a possibility.  What about USA information on Monarch numbers and
movements this year?  Does it look likely that one could have crossed the
Atlantic in the last week or two?  

Any information gratefully recieved.  

John Murray