not that one, but I did receive a different one.
Don't respond by email. Only by letter to the address of the real company, which you can look up on the web. Just in case the address or email on the letter is a fake. Quote their ref no and date of letter. They have several offices, one is in Lancing.
You need not enclose copy id documents in your first letter, just give your current and previous address and say that you own/used to own some shares in company x.
Equinity itself is a sound company, as long as you verify you are dealing with them, and not some scammer.