18 week old feeding habits - advice please

[possum18]

I have 18 week old twin boys, they arrived at 36+6 so not really premature but they are still classed as preterm because they didn't make the extra day
When they get weighed they correct their age by 3 weeks so this week they were plotted on the chart as 15 weeks.
They have always been small and never had any concerns, they continue to gain weight, however they have both dropped 1 percentile in the last few weeks. They are still feeding absolutely fine so not sure what to do - the HV made out it was terrible and she would have to come back first thing Monday to monitor them.

They are both over 12lbs and developing really well, loads of wet and dirty nappies and they sleep from 10pm-7am, she suggested adding a night feed - but I really don't want to disturb them.

They currently have 6/7oz at 7am, 1030am, 2pm, 530pm and 9pm. They never never cry for food and are always in a little milk daze after each feed and have a little nap.

What should I change?

Meant to say they were Breastfed for the first 8 weeks and now have Aptamil formula which they've been perfectly happy with

I wouldn't be wakening them during the night - if they were hungry they would wake. What centile are they on? My second is 11 months and not quite 17lbs yet, my first was 18lbs by 6 months! Both girls, less than 1lbs difference in their birth weights - they are just different! Even now at 11 months she will only usually take 5/6ozs at a time of milk (but is also on 3 meals a day now!) but at 18 weeks she probably only took 3/4 ozs a feed, although wasn't sleeping all night (and still isn't!) They are all different, DD2 is 9th centile for weight and 50th for length and mainly always has been and no one has even been concerned about her size! I guess if possible would they take an extra oz in each of their current feeds? I wouldn't be keen to disturb their good sleeping at 18 weeks!