It' called corrosion inhibitor, you can get it at plumbing trade counters.
It should come with the kit to put it in the system.
Easiest way to apply is to drain down system to allow a little air pocket in upper most radiator.
To drain down, turn off power supply, make sure cold feed to system is off; on vented system, turn off service valve to feed and expansion tank (in loft). On unvented make sure filling loop is closed, it should be already.
Then attach a hose with a jubilee clip to drain off valve which should be at low point on system, have hose running to drain outside or ground floor toilet or something - make sure there is an air gap between end of hose and drain (If you are draining whole system need to advice local water company, shouldn't need to do this as just draining a little bit).
Open drain valve, open bleed valves on upper most radiator to let air in.
Once air has been sucked in (rather than air/water comming out) to bleed valve for a wee while, turn off drain valve.
Then completley remove male part of bleed valve, fit attachement which came with inhibitor into this hole and inject the inhibitor. (read the bottle!)
Then refill system (vented = turn back on service valve to F+E), (unvented turn on filling loop 'til pressure dial on boiler is where it should be, then turn off again). Then bleed air out of radiator and repressure if neccessary.
If you don't understand any part of this... get a plumber to do it ( maybe a different plumber if the one you have been using is finding this taxing^^).
