Where should I go to read more about the Ordo Maximus and their possible connection to a London-area delta clinic? Does anyone know if there's any truth to the rumours that this clinic is capable of cybermancy? Has the Ordo Maximus been conclusively linked to any megacorp or national government?

IIRC, you can get info on the Ordo Maximus in the following books: "Threats" (the first one), a bit in "Prime Runners" (where Martin de Vries is described), and then there is also a secret conversation between two mages in "Cybertechnology" that, AFAIK, mentions the ability of the Ordo Maximus to use cybermancy.
AFAIK, the Ordo Maximus is not linked to any government and/or megacorp. They're just a bunch of humans with a hunger for power, and (in their inner circles) some mighty vampires with a wish to rule the world. The novel "The Terminus Experiment" has some more hints as to how they try to achieve their goals.

in short there's not too much officially on them, so feel free to do what you want based on a powerful cabal of corporate masters and vampires looking for world domination with a fondess for happy meals with legs.

Actually they have a relationship with the Villiers faction of Fuchi, which no doubt carried over to Novatech. Check out the 'Black Operations' adventure on page thirty-two of Blood in the Boardroom.

The adventure is set during the Corp War and Fuchi's break-up, and revolves around the runners being hired by the Yamana faction to go into the Villiers controlled Delta clinic located in Pueblo Corporate Council lands and take over and hold the place until reinforcements arrive. Seems the Ordo Maximus has been teaching the resident mages there cybermancy since after the takeover a Vampiric Mage from the Ordo called Monty Livingston turns up and tries to stop the runners/Yamana forces from taking over since they only have a deal with the Villiers faction.

And the cybermany part of Man & Machine, page fifty-four, mentions that the independent Delta clinic rumoured to be located in London is the only one that's been proven to have the expert magical knowledge needed for cybermancy. Couple that with the Ordo and I figure you've got a likely match. Plus, considering how regulated some things are the UK and horrendously expensive those places are to run - aside from massive amounts of wonga - you have to wonder what sort of protection they enjoy that allows them to keep operating. Possible relationship with the government in exchange for access? Or just enough influence gained through the Ordo's exclusive club front image.