Boundary line

Personally, I wouldn't be too bothered, as the neigbour has paid for what looks like a decent fence, for the sake of some posts that take up 4 inches every 6 foot of your property. I have a similar situation at the rear of my garden where the neigbour has taken it upon themselves to replace what is actually my fence with a better one. I'm not complaining, as it's saves me a couple of hundred quid and I now consider it their responsibility. (edited)

You've already had lots of good advice here OP but I would just add my support to the "it's best let sleeping dogs lie when it comes to boundaries" crowd.

You've got at the very worst, an incredibly minor issue with encroachment that totals - from what I can see - less than a square foot of your land. If you start to dispute these kind of scenarios, you can only lose long-term. Say you want to extend your house in the future, but you've put your neighbours nose out of joint over this minor issue, then you can absolutely guarantee an objection going in to the coucil. Also, if you make a fuss and then get the land registry documents out to prove your point, you may end up discovering that the lines were badly interpretted and they're actually entitled to shift their fence over three feet, for example. You have to ask yourself if it's worth it.

Furthermore, the pic you've posted seems to indicate it's a relatively new, solid and well put together fence. As others have said, that's something to be pleased with and almost certainly indicates it won't need maintenance for a good while and when it does, it's likely your neighbours will attend to their side at a minimum.

I'd personally go around and ask if it's ok to paint your side of the fence (it is, obviously, but courtesy will do you no harm) and just foster a good all round relationship with them that would be in jeopardy if you started to moan about the boundaries. Good luck with it.

I prefer to foster good relationships with neighbours. I would not be bothered by this in the slightest, in fact your side is now a blank canvas to design/develop (edited)

Having the posts on your land isn't a big issue as it isn't a whole fence so they have only taken space for the posts and at least they have put up a new fence as having an old fence is more of an eye sore

There is no legal obligation to have a fence at all, many properties don't. What if you kicking up a fuss results in him taking it down and saying put up your own fence? You've got a free secure boundry now, you should be buying him a drink!

Apparently there are no rules now whatsoever on who has the good or bad side. Seems more that the person paying chooses to have the good side, whereas years ago it was the other way round!

As stated above, having the posts on your side generally means that it’s your fence (so you could be responsible for renewal/repair etc). However in reality these are normally split

I'm in agreement with the others saying that it's not worth rocking the boat for a few centimetres... However, your neighbour has pulled a rum move putting the post on you land, facing your side. The post sides are generally regarded as the 'ugly' side and he knew full well what he was doing.