D
DeltaT
If you have plenty time just start from scratch as following the one pipe loop with another pipe will give a system with a far greater water content than necessary, therefore bigger bills.
Plan you new system and pipe runs. Have a google for 2 pipe system and work it out to suit your house. Run the new pipes up to front of the rads so they are easy picked up when ready. Follow what Dex says about sizing and reduce to 15mm for last 2 (maybe 3 depending on output) at end of runs. You won't need to bother too much with sizing for lengths if you have a normal size house.
If you are not too good with the copper and don't trust your soldering, use plastic. It is like Lego, even the weans could do it. You need copper for 1st meter or so from boiler and you should use it for any visible runs because plastic looks like a dog phishin in the snow (can still use plastic fittings or Cuprofits look better).
Fire in it is not hard to do.
Ah 'tamz', thanks mate you made me laugh. Not sure our 'little Engerlander chums' know what the weans are??!!
Reading your posts makes my life richer and reminds me why I love being Scottish............