What's the simplest way to go about this. Any particular recommendations
as
to different inhibitors.

Modern boilers will need something compatible with aluminium. I've used
Sentinel X100 in the past. I'm sure there are many more for you to choose
from if you prefer.

The problem with sealed systems is getting the stuff in there. Injection
syringes are expensive and can be messy. I tend to install an inhibitor
filling point. This is a short length of vertical pipe and an isolation
valve not at the bottom of the system. Just drain to well below the filling
point, open the valve and pour whatever chemicals you like in. Don't forget
to close the valve.

Christian.