Chris
I would contact them immediately to inform them that the second ESA50 has not been received. I suggest you confirm the address they are sending it to, even a minor mistake in house number or postcode could mean it is being delivered to the wrong address.
Having spoken to the DWP, I would follow up the call with a letter detailing the two missing ESA50s and your latest conversation.
Unfortunately, the DWPs default position is that if they've sent it, you have received it, so you need to put a stake in the ground now, to prevent problems later in your claim.
There is a downloadable copy of the ESA50 on the following link.
Use Of The Interactive PDF Version Of The ESA50 Questionnaire I would recommend you start completing this using the guides on the following link
www.benefitsandwork.co.uk/help-for-claimants/esa So that you can return a copy as soon as you know where to send it (assuming the return date has lapsed).
Gordon
