Mr E......I have just done the exact same on my CH.
I used Fernox DS40.....left it in for nearly a week, and blimey what a difference it made.
Millwall said it was good stuff and how right he was

Points to note about DS40.....
1.Dont leave it in for more than 4 hours if any part of the system is aluminium.
2.You will have to bleed the rads on a daily basis (tho this is a good indication that the DS40 is doing its jobs at dissolving the limescale)
3.Do not use DS40 if your system is prone to leaks (mine wasnt so that was ok)
One thing i found out, but is probably obvious....
When draining the system down, unless you undo the bleed valves on the rads (to let the air in) the murky water stays in them.
So the process i used to drain the system down.
Let the water out.....when it stops then undo the bleed valves on the upstairs rads....when that stops coming out....undo the bleed valves of the rads downstairs.....dont undo the downstairs ones first.....i made that mistake once

I filled, pumped, and drained it this way 4 times.....till the water was relatively clear.
Then i used Fernox Protector MB-1
HTH
