Hi xyz
You need to get onto the DWP immediately and say that you should have been given seven days' notice for an assessment. I would normally say ring the assessment services, but I assume they did not leave you a telephone number to phone back?
You need to ring urgently because you don't want to say that you haven't taken part in the process. I
With regard to changes since you returned the form, they can only take a note of what you wrote at that time. You would need to put in for a change of circumstances. You could say this when you ring and see if they will stop the process and give you the time to send in a new form.
Either way, you need to ring as a matter of urgency.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems