Good for you.... Expect a small written test on general ATPL stuff and your aptitude. After that, you might be required to play with their PS2 flight sim. You heard right, "PS2"....do one circuit and land. Then comes the interview, the guys are OK but do give them utmost respect. hint hint...
The rest will be briefed to you when you get there and, when in doubt ASK. Thats the reason they are there. But do your research on the company and the middle management (FOM, CP Safety etc..)
Have fun
Jolly flying