I ordered my monthly supply yesterday lunchtime and it arrived about an hour ago.
They only delivery Tuesday-Friday and it;s usually next day (if you order before 2.30 i think).
You can introduce it slowly over a couple of days, mixing with their current food. They have advice on their website about that, and there's lots of threads on here about switching foods.
Do keep in mind though that you can only keep it in the fridge for 4 days once defrosted, so if you are slowly switching there is a chance you may have to throw out a tub before you've finished it, IYSWIM.
Good luck! Hope you get on well with it.
Bingley's having duck tonight.