When you say 'otherwise you will miss some vital measurement' -- which one do you have in mind?

When I wrote the piece I didn't have anything in mind, I was just conscious that unless it's a spam dunk conclusion there wasn't enough information to be certain.

If you want to determine the load at the room level (which is what is needed I think) you are going to need radiator size/type, flow temp and deltaT across the radiator.  You will also need know the duty cycle.  Probably this is needed (given the context, ie determining whether microbore has sufficient capacity) for the most lossy 2-3 rooms only, the others will probably be ok, but some back of the envelope calcs would be necessary to determine this.

The mock-up run will clarify if the pipework is fit for a heat pump, but not which nominal power output you will need. If you put gas consumption figures to the flow and outdoor temperatures tracking, you will get nice statistics helping you to design the system. This is something no professional can obtain without producing a quite crazy bill.

In principle you are almost certainly right, but in practice its going to need a lot of measurement and interpretation unless its a slam-dunk case which sods law says it wont be.  The experiment and its interpretation needs careful thought with reference to the physics.  Im not saying it shouldn't be done, but interpreting it meaningfully if it turns out to be marginal is non-trivial. 

My advice is think through the experimental method, interpretation and possible outcomes first otherwise you will miss some vital measurement.  I, for one, don't immediately know how to interpret several possible outcomes without measurements you are unlikely to make unless planned in advance, and I have a degree in physics

Right, JamesPa, they don't, but me being nosy .... wanted to see if there's a chance of getting away without radiator swaps .... optimistically assuming that the microbores will do.

Maybe, but a separate question. 

I find it helps to thing of heating systems in 4 stages:

(a) Heat demand (set by the fabric, desired room temperature and OAT),

(b) Heat output (set by average surface temperature of the emitters relative to the desired room temperature and their effective surface area).  

(c) heat transport (set by the mass flow rate in the pipework and deltaT across the emitters - individually and collectively) and 

(d) heat production (set by the boiler/heat pump capacity at the design OAT and FT). 

For satisfactory operation the specifications for b-d must be capable of being at least as much as a.  All four will, in operation, end up in balance one way or another, possibly averaged over time if the heat source is forced to cycle because it cant modulate far enough down to match the demand.

If you divide the problem this way you can partition it so that each stage can be individually assessed.

For most efficient operation and greatest comfort you want the lowest FT possible.  This leads to large emitters and low DT to satisfy (b).  This in turn leads to high flow rate to satisfy (c).  This in turn is why pipe diameter matters, because the mass flow rate is limited, in principle by pump capacity, but in practice in most cases by the maximum velocity of the water in the pipes before erosion and/or noise becomes a problem

Yes for sure, but large radiators dont change the mass flow calculations and thus the viability or not of the microbore.

Right, JamesPa, they don't, but me being nosy .... wanted to see if there's a chance of getting away without radiator swaps .... optimistically assuming that the microbores will do.

Nicely large radiators -- single layer or double? With zig-zag sheet metal dissipators on their back?

Yes for sure, but large radiators dont change the mass flow calculations and thus the viability or not of the microbore.