Don't know if they're true or not but if they are, they're SPOT on with some of my theories.
Highlight if you want to see.
Mr Saxon IS the Master, but not as straightforward as you might think. Here are the key plot components I was told from forthcoming episodes:
1) In a brief but touching scene before the climactic episodes of the season, Martha wanders the Tardis (first proper inside-the-Tardis scene apparently) and spots a snapshot of the Ecclestone Doctor and Rose among other shots of the past Doctors earthbound companions in the Doctors "quarters" (brief glimpses of photo's of the Brig, Unit, Sarah Jane, Jo, Harry, etc). She asks the Doctor who the couple are. The Doc says its him and Rose. Martha can't understand how as he looks like someone else. This prompts a potted explanation of regeneration by the Doc - importantly revealing that he has 12 regenerations therefore 13 lives, as do all Time Lords (nice bit of continuity)...
2) Some time after this (not the same episode) near the climax of the series the Doctor identifies Saxon as a Time Lord - the sonic screwdriver scans his 2 hearts and other proof of Time Lord physiology. BUT, Saxon announces he is the Master and the Doctor exclaims it cannot be as he has already used his 13 lives and has survived by stealing the bodies of humans and therefore can no longer be a proper Time Lord (hence the point of the earlier scene described above helping explain this for the new younger audience).
3) Near the end of the final episode it emerges that the reason the Master/Saxon is a Time Lord after all is that in the future he has stolen the body of the Doctor's next incarnation and returned to set a trap and destroy him once and for all, changing the course of time and continue as a time Lord in the Doctors rightful body.
4) The Doctor ends up defeating the Master by outwitting him using the Tardis (not quite sure how) and changing the timeline such that the Master is defeated. But the Doctor is seriously injured in the process (climactic battle on a hilltop??) and regenerates into - guess who - John Simm!!
5) The new Doctor and Martha's first task in a brief couple of scenes after the climactic events, is to travel back in the Tardis to the point before Simms Master previously started infiltrating the Politics of Britain to eventually become PM, but doing something that stops this from happening, and so all peoples knowledge of Mr Saxon is prevented from happening, and the Doctor and Martha can travel the Universe as if nothing happened.
6) However, the audience is left with a teaser that Simms Master / Alternative Doctor may have esacaped destruction, setting up another dramatic twist that in series 4 the Master will return as the Doctors doppleganger. But not only that - as has already been speculated - the Master is indeed the Doctors brother, so he is set up in series 4 to be, by a temporal anomaly, the doctors twin brother!! Roll on!
PS. I asked if it is revealed that the Master and the Time Meddling Monk are one and the same as the name Harold Saxon seems to be a clue. My insider friend knew nothing of this, but confirmed that the Mister Saxon = Master no six anagram was deliberate. However, there is apparently a joke refeering to ex Labour PM Harold Wilson in the script that the older viewers will get, hence the shared fist name.