Technically, they're not trespassers until you ask them to leave and they refuse.
Steve.
not so - legally the act of tresspass is commiting any act on private land for which you don't have permission.
It is not just being on the land , although that is often how the lay person perceives it - so for example someone who is cycling on a footpath without consent is commiting the act of trespass regardless of whether a landowner stops them.
The really sailent point in practical terms is that trespass is a civil offence not a crime* - in one way this is good because it means that threats to report you to the police are just hot air (unless you refuse to leave/stop the activity - at which point they can ask the police for assistance in removing/stopping you), the landowner would have to take you to court which means both knowing who you are, and being able to demonstrate that you were in fact commiting trespass.
However if the landowner does decide to take a civil suit the bad things are that a) the burden of proof is lower than in crimnal court - it is a balance of probabilities thing rather than beyond reasonable doubt , and b) a suit can lead to damages although to get anything large the landowner would have to demonstrate not only that you were trespassing, but that by doing so you had caused him significant harm, nuisance, or expense.
* the addendums to this are that there are three (in fact there are others but its highly technical and not worth getting into here) sorts of trespass which are criminal offences
a) Agravated trespass - this basicallly means comitting a civil act of trespass but then agravating it by commiting another offence while trespassing - common agravating occurences are things like criminal damage - while photographing somewhere you shouldn't be, you cut/break a fence or tear down a sign , assault/affray e.g when asked to stop taking pictures you get arsey and get into a pushing and shoving match, or harrasment - this tends to be the preserve of paps but anyone could fall foul if they were stupid, e.g while on a footpath you repeatedly photograph someone or their property despite requests to stop.
b) Armed Trespass - much more serious in the eyes of the law. This is basically commiting an act of trespass while in possession of a weapon or anything which might be considered a weapon or used as such - while the obvious use is against poachers/badger baiters and such we all have to be careful of it as the weapons list would also include any knife with a blade longer than 2.5 inches , or any locking knife , it can also include anything you use as a weapon - so if while getting arsey with the landowner you threaten him with your monopod it could then be considered a weapon and you'd be heavily in the crap
c) Trespass on MoD land, Govt installations (a long list and not just the obvious) or anywhere with national security implications (an even longer list), this is extremly serious **** , and the sailent point to remember is its not just about being on the land - some MOD land is crossed by footpaths for example, but as soon as you stop to take photos you are trespassing and this may (although again equally may not) become a singularly serious offence.
at the bottom line four steps to avoiding getting into this legal minefield in the firstplace
1) Know your rights ,
including knowing what rights you don't have
2) be responsible - dont stray miles off a path on private land without permission, dont obstruct the path, don't take pictures through peoples windows etc
3) Know where you are, and if you are indeed on private land - and if you are and you want to do a lot of photography or any kind of comercial photography find the landowner or his agent and ask permission.
4) If you are photographing on private land (including on a right of way) and the landowner asks you to cease just do so - don't get into a big shouty argument insisting on rights you don't actually have (see 1 above). Acting like a prat doesnt help you, and also doesnt help other photographers in the future