what tank in my attic does cleaning stuff go into for cleaning sludge from radiators

Much easier and simpler to isolate a 1st floor radiator at both valves (bathroom radiator is best) drain radiator in to a suitable container at the radiator union, tighten radiator union then introduce your chemicals at the blanking plug on top, open the radiator vent with a key at the opposing side this will allow you to fill the radiator much easier. open rad valves then bleed radiator. This minor works should be carried out whilst the boiler is turned off. See youtube tutorial as a guide: https://www.youtube.com/watch?v=WouU0IKre30

if your system is sledged up then its best to have it completely drained down, flushed through, even taking all rads off and jetting out has sludge will trap in the bottom, then think about inhibitor and topping system back up.

It should be the smaller tank. but be very careful as the other will feed hot water, you wouldn't want to be drinking or bathing in the chemicals. If you turn on your hot water tap the other larger tank should start to refill, this way you will be 100% sure.

smallest one ,whatever you do don't put it in the big tank,will need to drain a couple of buckets of water to get the chemicals into the system

The Small one

The small tank usually ,the large tank is cold gravity to hot water tank and bathroom feeds.

feed and expansion tank - is small tank.