Stuck in London on a two day Finance Course (yuck), blue skies and September sunshine warranted a tube ride to Hampstead and a walk over the heath. The barrow is sadly fenced off, but is very large and prominent. It's covered with trees and surrounded by vegetation, so the full form is not easy to see. There may be a ditch around the barrow.
As mentioned below, it's well worth the trip on a nice day for the views from Parliament Hill.
Tom Graves describes a modern day 'retribution' story:
.. in the case of some barrows a thunderstorm followed within hours or minutes of the opening of the barrow. The same coicidence still occurs from time to time, as happened when a barrow on Parliament Hill in north London was opened recently; and I've heard that it is apparently a respectable piece of professional lore amongst present-day archaeologists. What is not respectable is to suggest that there might be a causal link between the breaching of the barrow and the thunderstorm that followed.
He goes on to suggest that the effect could be 'exactly like short-circuiting some kind of 'thunderstorm capacitor'. From p86 of his book on dowsing, 'Needles of Stone Revisited' (1986), which is actually free to download here: http://www.tomgraves.eu/needles