Whilst searching for a new angle on Tower Bridge the other night, came across this lovely setting by a bridge around St. Katherine's docks, very picturesque, barely got the pod out, cam mounted and started snapping before security appeared and told me it was private, I was being monitored and did I have a permit, I cannot use a tripod.
When i asked what is private, he said everything, and flung his arms about. Not wanting to draw attention I packed up and left, but how do you know they are not just saying that, there are no signs, he didn't provide any proof. Its for personal use, so going to the land registry is not viable, what is the general consensus in these situations?

From what I understand I can take a photo of anything providing I am standing on public land, but how do I know its public.

There are a lot of places around London that are private land, the area by More London (just down from City Hall), County Hall / London Eye and Canary Wharf are just a few that spring to mind. I've taken pictures using a tripod at these locations in the past with out being hassled, but I know others that have been moved on. If you do a google search there is quite a lot of discussion and info on it.
Personally if think there will be an issue, I do a quick recce, I get the camera set up on the tripod with the settings dialed in before quickly moving in setting up the shot and moving on before security strike.

If you lot get me arrested

Decided to go back and play hide and seek with security and practice my low light stuff, lovely place, just got back and pleased some didn't come out too bad.

A what? Google, google, google, ah I see, the Freeman 26 cruiser derived from a 22, this one looks much nicer than the ones on google images.

So what was he doing sitting outside golden wonder for then anyway? Apart from causing a nusience for no apparent reason other than because he could...

Irrelevant, he may have any number of reasons. Many of our best historical images are drawn from the archives of long dead anateur photographers and cinematographers. In contemporary terms those images would have seemed boring and pointless. 50-60 years on they provide useful insights into every day life.

Well done that man for standing his ground <time everyone did>!

That bit I'm OK with

It'd be the asking not to for me - I'd like to know why

We will have to agree to disagree, the only Tits evident in that were the jobsworths thinking they know the "law".

He is on public land undertaking a lawful activity, I anticipate their security cameras were filming him did they declare that to him?

<They are required to btw if he was on public land and believe an offence may have been commited in order to use it in court>.
As I said photographing and filming industrial scenes is a long standing activity for many "niche" photographers, for all manner of reasons. They made their point at the outset why "harrass" the guy after that?

IF you were that close to the water inside the St Katherine's dock area I suspect you were on private land (alongside private water). That area is very picturesque as your pictures show and is populated by masses of office workers and tourists during the week, especially lunchtimes and early evenings for the cafes and bars. I've seen loads of tourists taking snappy snaps without any problem but that's because they're holiday snaps. A guy with gear and kit setting up to take photographs of the boats of the rich and/or famous is a different matter. Personally I'm not surprised you were moved, especially given that their photography policy is in the public domain http://www.skdocks.co.uk/events-filming/

the problem can be that the boundary is marked solely by Studs in the pavement, especially if a developer promised road and path improvements as part of the scheme ...