I think the "tall" towel rads are probably at the root of your problem.
There should be a fair distance from the highest point in the circuit to the F&E tank greater than the head of the pump.
This creates a positive head of water sufficient to counteract the suction effect which the pump has on a conciderable lenth of circuit, which would otherwise encourage air infiltration.(as in your situation).
Usually a height of 600-700mm from floor to top of highest rad in circuit for the above conditions to be satisfied.
If the pump has "head" adjustment, this may solve your problem.(lower head setting)
If it hasn't got or doesn't solve, then other than fitting lower towel rads, the pump would have to be fitted in the flow pipe and not in the retun pipe as it probably is presently.
The the tanks would have to be raised in the loft.
It may be a matter of how the system is circulated. Are there circulators? Or is the system a gravity feed? Are other radiators on the same loop or is this one all by itself? It may also be a matter of pressure however, I assume that the sunroom is on the main level and that the ones on the upper level are running fine. That being the case then pressure is probably OK. Yet for every 2.3' of elevation you need 1PSI of operating pressure (+3PSI). 10PSI seems a smige low. 10PSI would suggest that the upper level (fan coil in the attic?) is 20' above the boiler.
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