I have taken a T from the supply and return to the nearest house radiator, not from the boiler although presumably these pipes come from and go back to the boiler.
When I installed the new radiator I arranged it so that the Danfoss valve was on the supply side (as I thought). I had assumed that the house radiator had been installed so that its valve was on the supply side, however last night after turning on the system I noticed that the return pipe from the house radiator was hot whereas the pipe to the valve was cold, so it may be that I have put the Danfoss on the wrong side of the new radiator, however the leaflet that comes with the valve says it can be fitted to either side and I have therefore left it where it is, but experimented with the adjustable flow control bit of it, without seeing any difference.
As I already said, I had turned off the house radiator in order to try to get more flow to the garage, but to no avail. This I did not understand since the house radiator pipes were hot, and shutting off that radiator should have meant all the flow was now going down to and coming back from the garage through the 10mm branches of the T couplings in the pipes to the house radiator.
The garage is uphill about 2 or 3 feet from the house radiator and I obviously got some air at the bleed valve when I initially filled the system. Since then I have bled about 3 litres of water from the bleed screw in order to try to get warm water through it, but got no more air (and no hot water!).
The Danfoss valve itself has quite a restricted passage through it, so I doubt if it would pass much more than the 7mm bore of the 10mm pipe. I gather that 10mm is frequently used in modern installations anyway, so am puzzled as to why it doesn't seem to work for me.
It's not a pressurised system- there is a header tank in the house loft.
If I need to take a supply from the boiler this would be very difficult and would involve long pipe runs as the boiler is in the kitchen which has a concrete floor whereas the pipes to the room radiators run under the floorboards. I can see no easy way of accessing the boiler direct from the room where the pipes go to the garage. I do not know how the original installation gets from the kitchen/boiler to the room radiators, perhaps bored through brick walls below the floor of the adjacent room straight into the kitchen, although the kitchen concrete floor is on the same level as the room wooden floors?
I'll open the pipes at the garage radiator as you have asked, but since I am getting water but not any air at the garage radiator I'm not sure what this will achieve, however I'll certainly try it later on this evening and see if I can tempt some hot water to arrive in the garage!