Orpheus in the Undergrowth

No, this was the episode that takes place in the private gardens of that apartment building. I know which episode you're talking about though, and it did seem odd that they were the only ones in park.

There are several private gardens in monied areas in London and yes, they are really quite peaceful during large chunks of the day. You are only allowed entrance if you are resident of the square around which the garden is set. This is by key only.

I filmed an episode of Panorama in a Holland Park garden square recently and it was empty for hours on end!

I guess people work long hours to pay to live on such squares in the first place so don't get much chance to enjoy them...

EDIT:

I've since had the chance to re-watch all the episodes and I believe the episode you might be referring to is Three Legs Good which is set in Regent's Park, which is just off the Marylebone Road in Central London. And yes, it is very quiet in the episode. I used to regularly walk to work through Regent's Park and it's generally fairly busy with people jogging, usually a group of people playing football somewhere, couples and groups picnicking, lounging and reading, Mums/Nannies with children playing, groups of schoolchildren doing art, the usual kind of thing.

I can only guess they emptied the Park [or a section of it] for the filming which is a bit odd... They could have paid extras to do the activities mentioned above.