The Daily Echo is reporting the confirmation that Bluestar will be taking on the contracts for what are currently Xelabus X11 and X12 from 28th October.
Source: https://www.dailyecho.co.uk/news/24622419.bluestar-taking-bus-routes-lordshill-shirley/
Interesting... I guess they were anticipating this some time ago, hence the reason why they went from 20 to 23 when they took on the Eastleigh services and missed out 21 and 22.
Presumably resourcing the vehicles will be no problem as there are a lot of vehicles parked up in the city centre between school/college peaks.
Back to the current Sunday workings, I do note that in some hours the 15 arrival at Central Station at xx50 does then go empty to the city centre to form the xx55 on the 14 just five minutes later. Seems incredibly tight, but I've seen it twice for real now (on one occasion, the 14 did indeed depart late) so I'm presuming that is the booked working. Specifically this definitely happens with the 1550 arrival forming the 1555 out.