David,
You don’t need an expensive or specialist pump - but you do need a pump capable of filling and pressurising the system. Ebay or Machine Mart are good sources for cheap single use pumps.
First recheck the hydraulic configuration of the circuit to ensure that you have the correct flow rates and no gravity circulation (unless designed to be so).
Ideally you need to know the system volume - it takes the guess work out of mixing and filling.
In the winter I pressure test the circuit with air at 0.5bar for 15mins to ensure that the circuit is tight. It does not work in the summer - the air expands too quickly.
Charge and check the airside of the expansion vessel to the correct pressure ( use a compressor or bicycle pump.
You need a decent pump two large buckets and three hoses (washing machine type) to flush and fill the water/ glycol side of the circuit. The system needs to have a pressure gauge readable from the pump point.
Connect flow, return and the pump to the permanent pump station. Connect the third pipe to the flush drain point.
Flush the system at least twice with a 10 percent glycol (don't use car antifreeze). There is always a risk of freezing any dead legs if you flush with just water in winter.
After flushing, fill with the correct mix and pressurise to the correct level to suit the expansion vessel with the pump.
Do not try to pressurise the water/glycol side with an air pump.
Do not try to use the permanent pumps to fill and pressurise the water /glycol side.
Close valves, check for leaks and commission the permanent pump(s)
Be aware that the Secon Triple line will only operate with the Secon controller ( an MRW I think) - unless the design has changed.