I thought a six month post-vaccine check-up was a bit pointless, but I'm really glad we went. The vet nurse said she'd give Delta a body condition score of 5.5/9, meaning he's carrying a little extra fat on him but not categorised as overweight yet. She didn't advise weight loss but emphasised not gaining more was important. Meanwhile Seb is a larger breed, so even though he actually weighs the same his body condition is ideal - he often cries for extra food at night. I asked if switching to an adult food early was a good idea, but she advised they stay on kitten food because Seb still needs it.
So of course I come home and start wondering how to address feeding two cats whose needs may be changing. Then today I found the issue. Delta is eating Seb's food. Delta finished his in the time it took Seb to eat half and booted him out the bowl, and Seb being the sweetheart that he is he just lets him.

Upside, supervised meal times are easier to enforce than different diets. So I'm going to see how this goes and make alterations if Delta's weight still goes up. The vet nurse said she felt mean telling me about Delta's weight, as a lot of pet owners don't want to hear it. I immediately thanked her for saying something because if there is concern about his weight it's easier to correct it now than later.
So long story short: the check-up was worth it for that information alone. I'm glad I didn't ignore the email saying they were due a check-up, as otherwise he wouldn't have been seen until their annual vaccine in November. Given he is only 9 months old now, 6 months could make a big difference.
She also booked their vaccine appointment for me, so I don't need to worry about making that six months from now.