The Last Sleep of Arthur in Avalon

[Ti Cl Nero]

Indeed, but how long was that kept up

I guess this is heavily depending on the context, but the few clear examples of this type of ethnical communities we have, didn't last more than a century or so ( for instance the Syrian cohors at Intercisa, Pannonia, around which a Syirian closed community arose, keeping ethincal connotation unchanged during the whole III century AD - see Fitz "Les Syriens à Intercisa", Bruxelles, 1972)
It is absolutely unlikely that such communities could keep on for 3 centuries - and this would apply to "british" Sarmatians as well, I think

But in my opinion, this truckload of Sarmatians (even if it was a BIG truckload) was the last to be transported to Britain. the next batches were settled in Gaul and northern Italy.

I agree.
The truckload to Britain happens in anomalous times for what relates settlement of barbarians - the reign of Marcus Aurelius (by the way, the surprising numerus gothorum gentilorum in Arabia dates from the same period) - and seems to be unique.
We know of at least two significant settlements of Sarmatians within the empire in IV century.
The first occurred during Constantine reign, and followed the uprising of the Limigantes which succesfully ruled out leading sarmatian groups forcing them to seek refuge in the roman empire.
Contemporary sources (Eusebius, Anonymus Valesianus) state that they were settled in Scythia, Macedonia, Italy and Thracia, and that those who were suited, were recruited in the army.
This *may* be where Alae Sarmatarum ,one of which we know to have been in Egypt in late IV century, may have originated (ND lists only VII Ala Sarmatarum, which suggests that 6 more units like that may have been recruited and then disbanded before late IV century).
ND lists also 15 praefecti Sarmatarum gentilium in Italy - which are commonly considered on the same level of the praefecti laetorum listed in Gaul: institutions managed by military, aimed to the settlements of large group of immigrates with the same pattern which had been used before in order to settle large groups of roman rempatriated pows when treaties were stipulated.
Likely, then, most (even though not necessariy all of them) of these 15 "italian" offices for the settlement of Sarmatians date back to Constantine - concordance with other literary sources is clear.
Interestingly enough, the ND also lists 6 more Praefecti Sarmatarum gentilium in Gaul, but sources never mention Gaul when talking about Constantine's settlements of Sarmatians.
So, these praefectures were probably set up after Constantine and before the end of IV century.
Sarmatians' pressure on the limes had grown again a few years later, and Constantius II tried to settle things with a succesful sudden campaign against Sarmatians who hadn't yet found refuge within the empire; he also sent most of the Sarmatians formerly settled by Constantine, back to their homeland (here is where some of the Alae Sarmataurm *may* have been disbanded) and tried to re-organize them *there*, also giving them a king.
Ammianus tells about these events (XVII.12).
Limigantes refused to accept those groups back, and here is where Constantius moved Limigantes far from the limes (still outside the empire) and crushed them when they tried to come back and revolt.
According to Ammianus, some groups of Limigantes, though, begged to be accepted within the empire as well, during these events (late 350s), even in areas far away from the Limes.
If this was at least partially granted (quite possible), the 6 praefecti sarmatarum gentilium in Gaul may have originated here.
That would fit : Gaul is far enough from that section of the limes, and no Sarmatian had been settled there before (according to the sources previusoly mentioned) - quite an understandable choice, then, since Sarmatian group settled before were mortal enemies of the Limigantes.
Anyway, we can't be sure things went this way, but we know that Sarmatians were in Gaul in late 360s: Ausonius mentions meeting Sarmatians coloni recently settled close to Strasbourg (Mosell.9): not only Constantius, then, but also Valentinianus or Gratian may have been responsible of these settlements of Sarmatians in Gaul - what we know for sure anyway, is that this settlement occurred right there.
We probably lack knowledge of many points, but I don't think we have clues of any other significant Sarmatian settlement within the empire before they disappeared - certainly it's highly unlikely that Sarmatians may have had anything to do with Britain again after their first settlement there by Marcus Aurelius - nothing at all allows us to suppose something like that.

Vale