home office interview with all dependents

[rioses]

I got a fee waiver for ECHR application.Whilst waiting for biometrics or possibly a decision,I receive a letter from HO to come for an interview with my dependents ,even to inform my children's school for permission to attend the interview.

I was previously sent form IS151a..NOTICE OF REMOVAL before my current application.

it states:

'the interview is required to collate and verify personal data about you and your family.You are required to bring any documents to proof your nationality with you.These documents required are like :

National passport - birth certificate - National ID - Military service card - Driving license (even if expired) - Educational certificate - letters from family back home.

It was signed by....RCC FAMILY RETURNS TEAM
APPEALS DIRECTORATE

Anybody got any idea why this?
How long does it take to receive letter for biometrics in fee waiver cases.its been about 5 weeks when i received the acknowledgement letter.