If you managed to get an appointment, they have feedback forms at the PEO (at least in Croydon). DH and I wrote as much as we could fit on there. We were sure to mention the online booking system.

Maybe be prepared with a list of things you want to address before going to an appointment.

If you give them your address, they send you a postcard thanking you.

I kind of want to use that as one piece of 'proof of cohabitation' for ILR.
