I encourage caching by public transport. Take the Northern Line to Golders Green, walking distance from there is roughly half a mile. Buses 210 and 268 take you within a quarter mile of the cache (Stop North End Road / Hampstead Way).
Hampstead Heath
Hampstead Heath is North London's largest and probably most important green space. The wildest of London's parks has fields to run in, long grass to roll in, woods to hide in and, most famously, ponds to swim in. In north London this is THE place to get back to nature. Strewn with picnickers, cyclists, families and the rest, the heath is large enough and has enough copses, hills and mounds that a quiet spot is never far away.
The Extension
The Hampstead Heath Extension does not share the history of common and heathland of the rest of the Heath. Instead it was created out of farmland, largely due to the efforts of Henrietta Barnett who went on to found Hampstead Garden Suburb. Its farmland origins can still be seen - it preserves the old field boundaries, hedgerows and trees.
There are a couple of caches further north in the Extension that are worth a visit too (not part of this series though).
The Cache
You're looking for a slightly camouflaged, regular size plastic box that will hold some swaps and TBs. Paths in the area can be muddy after rain. Stealth required, the cache is near the main path through the extension! Please take care to replace the cache so that it can't easily be seen from "outside".
Take note of the letter and number under the lid for the bonus cache.
Original Cache contents: Logbook+pen, some small gift items (a set of playing cards, an England keyring, a dolphin keyring and a heart keyring) and a TB.
Wheelchair Users
There's a relatively good path (but muddy when wet) passing at about 10m of the cache. The last part is a bit rough, uphill and over some roots, so a helper will probably be needed to retrieve the cache.