There will be instructions on the box regarding how long to leave the lawn treatment down before you cut the grass. It should be watered in well if no rain falls within 3 days, so I imagine that's why you watered it.

Scarification is actually best done nearer autumn than now - the mosskiller element in the treatment you used may take 2-4 weeks to blacken the moss in your lawn. Once its gone black, you can rake it out, but the effects of the lawn treatment you used will remain for 6 weeks, so you should certainly wait that long before you scarify. When you do scarify, cut the lawn first, scarify, then cut again, because by the time you've finished, there will be long and tatty growths of grass sticking up in the air (hopefully). The lawn will look absolutely dreadful by the time you've finished - but it'll recover, although as I said, its best not done now, particularly while its so hot.

Certainly good advice, I have a problem in one part of my lawn which I think is because it doe's not dry out as it is a shaded part, I use Weed& Feed twice a year and it seems to keep it under control.