Ok, I've just bought an IR thermometer with laser guide (IR89 -
awesome) and have began balance my rads in earnest.

I have a choice of 2 rads which could be considered closest to the
bolier. One rad has a 4 degrees C drop across it (20-16) but the
other, get this, has an increase across it (21-27)? Have the builders
put the wheelhead/lockshield valves on the wrong way round? Would the
central heating system still work ok if this were the case?

Any advice greatly received as always.