ESA - no letter?

Evening,

I have been placed in the WRAG after having my WCA a few weeks a go. I found out because housing benefit wrote to me due to a change in my income and stated this was due to me being placed in the WRAG. I rang ESA and they confirmed it. Anyway, I was expecting that I'd get some sort of written confirmation, explaining their decision and time spans etc. However, I've not had anything from them and was wondering if this was normal? I could do with a letter explaining it all as proof.

TIA :)