If you search for 1200 x 600 modular T8 fluorescent then you will find most are 36W tubes. There is normally hardly enough room to get the tube in place.Well I did look into this and the 4ft lamps are around 1215mm in length, wouldn't this be to big for the 1200mm x 600mm module? Well the guy has changed the lamps he said and the starters and still doesn't work. Can only assume it's will be the ballast, there are 2 ballasts as you said 2 lamps per one, if they are 3ft 30watts each what wattage ballast would i need and same with the 4ft 36watts? (But surely the lamps would be to long for the fittings itself?
Magnetic chokes are becoming harder to get now but you would want a 36/40W choke like this.
For a twin system with HF electronic ballast you would be looking for a 2x36W electronic ballast like this, however they come in different sizes from different manufacturers and you would need to ensure you have space.
I do not know where you would get 2x30W ballasts but you can google it.
Check what you need first.