Part of the Great Bathroom Rebuild involves fitting a big new towel
rail, and this naturally involved breaking into the (sealed, combi)
central heating circuit. At the moment I just have two capped-off stubs
sticking out of the wall, ready to plumb in the rail/radiator this
weekend. For the couple of weeks since I fitted those stubs, I just
topped the system back up (didn't have to drain it fully down) via the
filling loop, but once I fit the radiator I'll want to properly
"recommission" the system with an appropriate additive.

I have no idea what was in there previously. I also think I might have a
small leak somewhere, as the system needs a little more topping up than
it probably should (not loads though). Note that this predates my work
on it! Can you get leak-sealing additives that might sort this? It's
possible (not having found dampness elsewhere) that the leak is in the
boiler, which is moderately decrepit and will probably get replaced in a
year or so, which is why I haven't worried about it too much.

Other than that, I just need a decent inhibitor, or whatever one
normally adds (not had to before). Any particular one recommended?

Finally, how do you actually get the stuff into the system? I've only
ever known my Dad's approach, of tipping it into the expansion tank of a
conventional system, but mine is sealed. I have a normal filling loop
under the boiler, and (luckily - I hear some people don't) an outside
drain cock. Is there perhaps some cunning fitting that you put inline
with the filling loop, to push a bottleful of gunk in ahead of the water?

Do I need to completely empty the system first or anything?

Any advice on getting the air out again afterwards? This towel rail will
be the highest point on the system, though I'm not sure everything will
necessarily run nicely upwards towards it.

Cheers,

Pete