Replace the pressure limiting thingamies with copper pipe. IIRC 15mm should see you right. If it doesn't, then by a process of elimination it's 22mm. Use jubilee clips as the spring clamps may not be the right size.
Make sure the heater is on hot, so the valve is open, put coolant in, squeeze top and bottom hoses and keep topping up when you start it AND when it's warmed up, so the stat has opened.
Air locking is pretty unlikely - it's not like it's a T25 with 16' of water pipes and about 16litres of coolant waiting to airlock, now is it?