It's more likely the Optare Solo SRs from Green 5/7/8/9 will replace the 7 OmniTowns dedicated to the 53/54, and should be able to oust the last 2 Optare Solos still in service (237/238).
I believe but can't remember that NCT actually have an order for 10 Enviro200MMCs, which could mean 2 could become spares.
If anything, its the 11 branded Optare Solo SRs that don't tend to stray off other routes; its just very likely some are off the road due to maintenance... One is in fact off the road following an RTC, hence why 3 out of 4 branded buses have been out recently.
On the Facebook saying the new buses are coming by the end of the month, makes it a bit more clear.
"
Nottingham City Transport Stefan Beck We have purchased 10 midi-buses because that’s how many we need to upgrade the midi-bus fleet to meet the clean air requirements from January and replace our oldest 10 buses, which are those on the 53, 54 and in our spare fleet."
Looks like they needed 10 new ones to meet Euro VI standards, no CNG midi-buses available and Electric too expensive at the moment. The 10 needed to be replaced are spares and the 53/54, so might as well upgrade buses used on well used routes with the new ones, and move their old 10 buses to the less used buses; will also mean the fleet is entirely double deck and Midi Bus (no single deckers).
They also tweeted a link to an interview with there Engineering Director
https://open.spotify.com/show/6CGUnevXFpEupqO7plgwxH which makes it sound like there won't be any new buses for a couple of years, but then expectation is any new buses will be CNG for Double Deckers, and electric for the Midi buses.
And the 11 now seems to be running mostly with branded buses again.
