You can't determine if a pump is operating correctly by its temperature.
Don't think you will be provided with any differential pressures before/after a power wash.
Others will inform you but IMO a power wash will not clean a blocked Hx.
Don't know if you can actually hire/get a clamp on power measurement meter that gives you watts , it may only give the current in amps which is only of limited value.
Because you didn't or still don't really know if you have a pump or a restriction problem you were able to determine reasonably well IMO from the boiler output and the deltaT across it that the flow rate (by calculation), appears to be between 3 and 4 LPM, say 3.5 LPM, this information, on its own does not tell you that you may have a restriction or not as the flow depends on the pump head which in turn can be read off the pump curves.
Flow rate is 3.5 LPM or 0.21 ms/hr (3.5/16.666)
To read off the pump head, go to 0.21 on the horizontal (X) axis and "draw a line" vertically upwards until it meets the various speed curves and the corresponding head is on the vertical (Y) axis. You can see that IF the pump is running on speed1 the head is 2.9M, if running in speed2, 4.6M and running in speed3, 5.5M.
You havn't definitely confirmed that the pump is running in speed3 (you might do so) but even if it is only running in speed1 you will still only get 0.29 ms/hr, 4.8 LPM @ ~ 5.4M head on speed3 because flow is proportional to the sq.root of head so again this would point to a restriction somewhere IF the pump is operating correctly.
We can talk about it until the cows come home, if it were my system I would just go away and install a 6M A rated pump that displays the power in watts and you will know almost instantly where the problem is and while it will not fix it if the original pump was OK, at least you now know where the problem may be.
Any thoughts on the heat I mentioned of the pump front ? If I paid for a power flush, would a differential pressure figure be given before and after ? To see if cleaning helped, would. A power flush clean the HX ? and or if there is a problem.***.how do you determine the amount of head required ? Is it from the boiler or pump curves not familiar how to do this.
if I pay to hire a power meter, will that be of any benefit ? Thanks
Also, could piping configuration, particularly changes made during the extension build be responsible for this restriction or could it only be solids build up ?