Yea, I thought about that some before I responded, definately parallel. In fact, at one point I shut all the downstairs rads in the same circuit (to continue our analogy) off and did get hot water to the upstairs rad, it didn't seem to blow a clog though since after I turned them back on, no hot H20 got upstairs.
I had one idea after reading your post yesterday, I wired up the circulating pump to run all the time (previously it ran when the boiler was on). Didn't seem to make any difference, I'll probably undo this 'fix' tonight.
Thanks for your help and please give me any other ideas you may have.
It my be releveant that my Mother-In-Law lives up there so there is some interest it keeping her warm. She's using an electric radiator now which is killing my electric bill.