Hi Sauce,
Everyone now has to take the online SHL tests before stage 1 (this is what I was told by a lady from HR at my stage 1 at the beginning of September) so its nothing to worry about. When I did mine there were two online tests:
1. checking test - you are given a string of numbers or letters and have to pick that same string out of a list of similar multiple choice answers. This is timed and towards the end of the test you have to work very quickly (only a few seconds per questions).
2. maths test - fairly standard maths questions, involves rearranging equations to get the answer. Again this is timed and gets quite tight towards the end of the test.