Worth flushing the new pipework before connecting to the rest of the system. Sentinel tells me that flux (and oils inside the new radiator) often remains in the system and the dissolved copper it holds can actually prevent inhibitor from working correctly. And the oils can damage EPDM rubber in systems - quite a bit of rubber in some Valliant boiler models.
Best practice is to fill the whole system with new systems cleanser to get the flux out, circulate as per manufacturer's instructions and then drain. It is doubtful how many people will go to that length, particularly for a small modification such as yours, but if the labour's free, it's only the cost of the F3/X300 (etc.). Many people would be happy with an old-school cold flush, then hot flush, and then refill (i.e. refill the system and check for leaks, circulate the water and drain, refill again heat the water and circulate and drain, and finally add inhibitor and refill) and hope that gets most of the flux etc out.
You can use a hose to siphon the crud out of the F & E rather than drain it into the system. It can be one of the most satisfying plumbing tasks.