Just keep going Brinny! Both caps off until the wing tank overflows, then replace that cap and keep filling, rad bleed bung out. When the bleed bung hole runs without bubbles, replace the bung and top up the central spout. When starting it, pointing the front uphill helps, but you MUST put the heater control to defrost before starting. This ensures coolant goes through the heater and helps fill the system generally. Once it has got semi-warm. stop it and remove the radtop bleed threaded bung (LHS of rad) and top up though the central filler spout until it runs out of the bleed hole. The replace bung top up central spout and restart. it is quite normal to have quite a few heat and cool cycles before all the air is out.