I can really recommend two sites, firstly Wint has already identified Slufters pond as a good site for Black Darter, I saw plenty here last August, plus its also a very good site for Emperors as I remember.
If you are visiting the New Forest then the ultimate site for Dragons & Damsels has to be the stream at Crockford Bridge SZ 3499, park in the carpark and then walk out of the back of the carpark out onto the open heath, head to the right around the edge of the wooded area until you find the stream then just explore the area.
I have visited countless Odonata hotspots up and down the UK over the last 15 years and this is definately the
best that I have seen, if you hit it on a warm sunny day it is truely awesome!
Specialist heath species present are Golden-Ringed Dragonfly, Southern Damselfly, Beautiful Demoiselle, Keeled Skimmer, Small Red Damselfly, Scarce Blue-Tailed Damselfly, as well as all the commoner odonata.
Added bonuses come in the form of Large Marsh Grasshopper,
Grayling,
Grass Snakes and
Common Lizards.