I imagine the extended XC services will arrive into Southampton platform 4 (xx17 is probably a PTT time, I imagine the WTT time will be more like xx13), then ECS out to either of the goods loops, then back into platform 1 ready for departure.
In the weeds of the report I'm about to mention they reckon the layover will be in the Up Goods Loop, but read on...
I found this detailed report online which went to the timetable disputes panel about this XC service. It is basically SN's complaints about NR rejecting their bid to run the Brighton to Southampton service in a loop via Eastleigh and the Airport after the morning peak.
In the details of XC's track access application, they refer to the dispute panel's decision being made in their favour, and to cut a long story short it seems that the XC service extension prevents SNs from being extended via the Airport. That is also a franchise commitment, but it seems NR have the final say so, and they've decided XC wins out on better passenger connectiviity.
It seems like the SN Southampton and SWT Poole services both needed the same path between Eastleigh and St Denys? As it stands now, the SWT Poole seems to arrive on P3A about XX13 just before the SN Brighton terminates in P2A at about XX15. What seems to be happening is that the SWT Poole service has been flexed to allow for the XC, presumably running just ahead or just behind.
There was a lot of other info in the report, but as far as I could see NR also won't allow most of the proposed SN services to reverse in Eastleigh's platforms as planned, because they'll be conflicting with freights most hours of the day.
Will be interesting to see how it all pans out once the new timetable is in the public planners (in about 3 weeks time?) - but nothing is ever as simple as you'd expect...
I haven't quoted directly from the report, as it's one of those pdfs that is restricted access, and a zipped file as well. If anyone wants to read it themselves it is the 'submission' linked alongside 'TTP 350' on this webpage here:
Access Disputes Panel