It's a 7 week pattern. Going to be difficult to show on here but bear with me.
Starting on a Monday. 5 Lates, 2 Rest Days, 4 Early Earlies, 3 Rest Days, 3 Late Lates, 2 Rest Days, 1 Early, 3 Middles, 2 Rest Days, 2 Early Earlies, 2 Earlies, 2 Rest Days, 4 Late Lates, 2 Rest Days, 3 Middles, 1 Middle or Late, 1 Training Day or Late, 1 Rest Day, 4 Earlies, 2 Rest Days.
The pattern repeats but there is also assigned leave which fits in after the 4 Earlies (but not always).
It's a bit complex. Serco are an OK company to work for. Could be better, could be worse. The union keeps them in check pretty well.