Since my Sinhala typing is slow and I don't have enough time today, I'm gonna write this in English.
I've been studying this group and their leader for sometime now. They've been operational in Sri Lanka for a long time (possibly since 1971, as they claim in their group). I don't know how or where it started, but it has grown slowly, and is operating as a "cult" of sorts.
I admit, I was curious about them when I heard about them. I don't believe in the traditional beliefs of an Arahat having to be a monk, as I believe that it is merely a state of mind and therefore can be achieved even by lay people.
I think the first time I got to know about their operation was around 2002 or so, when I met a lady who said that the path to Nirvana is not found in meditation but it must be understood "beyond the senses" (a common argument of the Satyagaveshi group and Dr. Senanayake). Basically, she claimed to have found a "shortcut to Nirvana", and told me that there are lay Arahats in Sri Lanka, and such persons are teaching all over the country. Being naive back then I really wanted to meet these people (who wouldn't?), but due to me being busy with school, I couldn't. I suppose I was lucky!
I have read their arguments and teachings from their facebook groups and have seen some of Mr. Siriwardena's videos. A Buddha is supposed to teach something people don't know about (which is what they claim as well, saying that their teachings are not found anywhere else). Alas, Satyagaveshi and Siriwardena fail at this miserably, as most (if not all) their teachings are copied of existing Buddhist teachings from all over the world.
The best example for this is "finding the Buddha within you". In Mahayana schools, this idea is widely taught as finding your "Buddha Nature", which refers to your ultimate state of enlightenment. They believe that all beings have the potential to become a Buddha. Keep in mind that rather than becoming Arahats, the Mahayana ideal is to be a Bodhisattva. In this sense, everyone can be a Buddha. Sorry Mr. Siriwardena, you've copied this one.
Also, the concepts of "looking beyond the six senses" and "looking beyond teachings and thought" are well known concepts taught in Zen Buddhism (which is quite cool, when you learn it PROPERLY). For example, the Zen state of "Mushin" is realized when the mind becomes devoid of thought and is completely receptive and clear (often pursued by martial artists). More plagiarism, Siri!
As for the Buddha existing "beyond time" and "beyond this world" as an "eternal", this idea too, is borrowed from Pure Land Buddhism, which speaks of the "Amitabha Buddha", who is a celestial being, eternal and enlightened (rather than our version of Buddha who has ceased to exist in any realm). Their idea of Nirvana is an eternal place. Well Mr. Siriwardena, you could have at least used citations!
Thus, it's pretty obvious that this Siriwardena fellow has gloriously combined random teachings from Theravada, Mainstream Mahayana, Pure Land and Zen. He has then sprinkled these teachings with his warped logic and "hidden meanings". The best example for this was "Asita's head scarf" story, which was in one of his videos. He takes the simple story of the infant Siddharta's placing of his feet upon the head of Asita as some warped out hidden story about exceeding knowledge.
Dude, I can do that to a random story too. So can anyone else.
The best part is, he's also using teachings from an organization called "Share International", headed by a misguided old man called Benjamin Creme. They believe that Maitreya is some sort of a world teacher that will come very soon into this world and lead people into the next stage of enlightenment (on a global scale!). The 2012 stories are also obviously nicked from conspiracy theories around the world....
In conclusion, either this guy is very confused, or he's confusing people for his own benefit. Sure his teachings have some sense, which makes it appealing to people. But not all his teachings, and that disqualifies him as a Buddha. Besides, why would the new Buddha need to teach through the old Buddha's teachings? Did Gautama Buddha teach using the previous Buddha's teachings? Quoting his scriptures etc? No, he did his own thing and taught people the simple truths of life.
And building 500 foot tall statues is certainly not helping in proving one's validity as a Buddha...