I am a first time SG, so I was adapting the Semita Errabunda setting and putting the entrance at the middle of London Bridge. Some of the abbreviated salient background story is as follows:
To explain my mage character, I am saying that he is from a long line of magi that have always been initiated inside the London walls, from the time they were wizard-priests of the temple of Mithra (so his Ex Misc free major Virtue is Ways of the Land - Towns). They have an un-official but traditional claim to all of London.
Prior to the Saga I have started, a secret Mercere covenant trying to pull itself out of Winter negotiated for the right to settle on the new stone bridge being built - one where they arranged to shift the starlings a few mystically significant paces so that the Sacred Geometry lined up with the elemental force of the Thames river, and boom, created access to a Magic Regio. They had just managed to move most of their stuff in when the 1212 fire broke out, and the paniced crowd did most of them in. The Covenant didn't bounce back.
Enter our troupe of newly minted Magi about to settle what is left behind.
Hereward the Hound Master (name simply popped into my head when I saw the map of the Isle of Dogs) is a descendent of one of the magi who tried to make the London Covenant (after being invited by my character's antecedent) a couple of centuries ago. Between Alfred the Great repopulating Londonburgh, and personality conflicts, the original London Covenant fell apart and Hereward's antecedent ended up in the Stepney Marshes (aka Isle of Dogs back when there were a lot of black poplars growing there) and they and their apprentice lineage have made their home there ever since, with a mixed relationship with my character's antecedents in London over the generations. Hereward is currently a bit miffed that he didn't get first right of settlement/looting of the "secret" Mercere Covenant.
The Mercere Covenant was a secret from just before the Schism War. Basically the leadership of House Mercere saw the negative publicity of people using Mercere Portals to raid Covenants, and fearing that a Grand Tribunal might outlaw Mercere Portals (the greatest secret of the House), they decided to pre-empt the enforcement of that possible ruling and hide their best Portal makers in a secret Covenant. Over the decades, the Covenant has been forgotten, forced to move, and lost its focus somewhat, though it made a secret Mercere Portal network and other stuff related to keeping the roads open for Magi (or at least Redcaps).
Thinking about what I have read of the history of the Isle of Dogs, and the various Magi my troupe have rolled up, and people's suggestions above, I now think Hereward could either be a Jerbiton playing a rustic lord, or perhaps a Mercere Mage, either of which who doesn't like to socialise. At any rate he started life with an odd set of supernatural leanings that his parens didn't erase, so now he is the "Hound Master".
Odd question - how likely is it for a Mercere magus to quit House Mercere and join House Jerbiton?