1) Yes, we usually tell people to expect the reasessment forms 3-4 months ahead of time.
2) You have 4 weeks to return the form, counting from the date on the letter - so the 21st Feb is correct in your case. I realise that, given the need to ensure that the form is received by ATOS in good time, this actually gives you more like 3 weeks, but I am afraid that is how it is