Just an update on this.
In the email address that I originally thought was associated with the account (and one which they deny is linked to the account) I have found a number of emails.
The first, in 2014 was a welcome to my wallet email. This has the wallet ID which matches what i had written down.
There's another in 2017 asking me to verify the email (whether I did or not I have no idea)
Then there were two emails in late 2017 asking about attempted log ins from a mobile phone.
A final one was sent in 2018 regarding EDPR updates.
So I have sent another message to blockchain customer support.
If you say the last message was in 2018 and you may not have logged in since then there is a high possibility that they deleted your account due to not logging in for a long time or due to new wallet updates.
Or there is another possibility, since you say that you logged in from a mobile phone, perhaps someone other than you logged into your account and changed the associated email.