Having googled this, I realise I should have figured out the flow and return of each rad before taking the system to pieces. Hindsight etc!
Unfortunately, the system is completely drained down, and all the old rads have been removed. So checking which side of each rad gets hot first could be a challenge!
However, access to almost all pipes under-floor is currently good, so I could "easily" do some tracing.
In a drained-down system, how can I tell, at the boiler, which is flow and which is return?
TIA
Cheers, David.