Check very carefully the warranty situation. You'll find more often than not that they've invalidated their warranty by not having the floor either sealed in the first place or not maintained regularly enough. It's pretty much a guarantee that the average customer has breached it in some way because they didn't read the small print.
In truth, if it's trashed you have to give it some welly. Strong, high alkaline floor stripper, scrub/dwell and then a real thorough rinse. Blue is the official coarsest grade of pad recommended but in the real world when the floor is already trashed you make your own judgement calls (I regularly use a black pad with no problems, but you need to be sure)
Karndean products are crap, use your own professional stripper of choice and Amtico dressing for a less shiny finish than professional vinyl polishes normally give (always ask customer what finish they want).
A scrubbing pad will ALWAYS get into scratches/grain a lot better than a brush. Why? Pull out one fibre form a scrubbing pad and compare it to the thickness of a bristle from your brush. Much thinner. When under compression it is forced into the scratches, and a lot more surface contact is made.
Science lesson over
