Unless you've broadcast the seed all over, you can mow the parts where the old lawn is growing, but the new bits need cutting with shears the first time, when its about 6 inches (unless you've got a good, sharp Flymo which does actually float properly), and this time of year, probably the second cut as well needs to be with shears. As for the treatment you've bought, a couple of months really, which effectively means you can't use it this year - I think July is the last month for feeding lawns (check the box) and after that it has to be an Autumn treatment which is a different chemical composition.

Thanks you for the quick reply. I had thought that I could use the treatment any time, should have read the instructions before buying!

It'll keep till next year provided you keep it somewhere dry. First treatment's April then another 6 weeks later.

Thanks again, I will maybe take it back and buy a plant instead, April is soooo far away!

Not at my age, it's not - I often feel its barely worth putting the Christmas decs away, it comes round again so fast, lol!

Same here Bamboo - its quite scarey.