Just been on break for 23 mins and had to put card into a truck which I’m about to drive in ten mins (company require this period).
Did a manual entry to record 23 mins break and then changed tacho mode to break for another 10 mins. If I’m stopped, will this count as a legitimate break?
No because putting the card in the truck would be other work therefore interrupting your break.
Edit.
The 23 minutes you had will obviously count as break so the 15 of that could be used as part of your required break obviously leaving you a minimum of 30 mins to still take to reset everything.
Ah, of course. I’ll have to take a break when I get back then as got plenty of time.
Did you do any driving before the 23 min break 
I presume the company will want your tachometer running on other work for 10mins, not on rest, before you move the truck to show you’ve done your walk around checks!