Moosehead,
Ahh, I'm not a plumber, but one of those dangerous people that knows a little bit.
The aerator (or a least my one) is about the size of a baked bean can and has the cold water (from the header tank) connected to it, as well as the hot water flow from the boiler (in on the cold water feed side and out the other). It also has the expansion pipe (running back to the top of the header tank) connected to it too.
In my case it was close to my boiler. The bolier a floor-stander, and is upstairs, with the header-tank directly above. The aerator is a ceiling level directly above the boiler, and directly below the header tank.
So, I guess in your case, if you trace the pipe runs from either the bolier or the header tank, until you find some thing that looks like a baked-bean can with pipes sticking out of it, you've found it.
Hears a tip:
If you system is really badly sludged up, not end of flushing and de-scaling is going to clear it properly.
What I did was to drain the system down, and cut the pipes around the aerator so that it could be withdrawn and (carefully) cleaned out by flushing and rodding with something like stiff electrical wire.
I also found the pipe from the boiler to the aerator particularly sludged up too, so I disconnected that end from the boiler at the same time and rodded/flushed that out too.
Then the whole lot was put back together using good old-fashioned compression joints where the pipes were cut, so that if it ever happened again (and it did 7 years later) it was an easier job to disconnect it all and flush it out.
So many people have helped me on this website, I hope this helps you.
Good luck.
Ian