Just to clarify, the online test is NOT the first assessment day or 'Stage 1' as its known. The online test is just a quick thing you do before you're even able to schedule the Stage 1 test day.
Basically, this is what I've done so far:
1. Complete online application - received very quickly a reply saying it'd been received and was being considered.
2. Received e-mail saying:
"Congratulations, you have been invited to attend the first part of the Recruitment Process - The Online Selection Tests."
3. Completed online test (15-20 minutes or thereabouts) and then immediately sent an e-mail to HR saying I'd done so.
4. Received e-mail saying:
"You have passed you online tests, I will move you onto the next stage
'invite to tests' but please be aware there will not be any dates
available until April but we will be putting them on the system in
February."
And that's it. All in all took just a few days.
So now I'm just waiting for the April Stage 1 test dates to be made available. But the first part of the process was very quick and painless for me at least.