Thanks guys - all useful stuff !
To be a little more specific about my problem the problem occurred with
two 22mm plastic pipes that run up the wall in the hall. There was leak
where they joined other pipework under the bathroom floor above. Whilst
I took up the tiles in the bathroom to gain access, my wife chopped out
the two plastic pipes all the way down to the under the hall
connections with the idea that I would then replace the plastic with
copper. However in the fun of the moment we didn't pay enough attention
to which pipe from below the hall joined to which pipe above the hall
ceiling (in the bathroom floor).
I have completed all the pipework now, re-filled the system and am now
running it up.
Apart from a lot of airlocks it appears to be working OK but I am still
concerned that I may have reversed those joints. The boiler (which is
situated 30 yards from the plumbing incident ) is firing up alright and
the correct outlet pipe (ie the flow) is getting hot first.
Would this have happened if I had incorrectly joined the pipes ?
Thanks again for all this valuable help.