The drain arrow simply shows where the hole is above the threads / under the seal ring. This is useful to drain some oil out into a little pot, before removing the filter. On yours though, as the filter looks about vertical, not much help. The o-ring is correct at the top.I am not clear where the plastic disc fits in picture two? I have strangely never worked on the later Vauxhalls, (that's what it is isn't it?). I have done filters of this type on many PSA and MB types, and you just firmly press the element into place on the lid and screw in. No loose parts. The fitting torque, is probably moulded into the lid, and is usually 25Nm. Over tightening is a real no no. Even if you have to buy another filter to get one, that o-ring is going to have to be replaced.