Sending but not receiving

[#1]

Set up a new mailbox on Wednesday - I can send e_Mails but when I reply to the same E-Mail (using GMail) nothing is coming back. GMail is not bouncing it so assume its wandering around
lost in Sheffield. I'm sending the original from plusnet Webmail, not from an EMail client. 

[#2]

You need to login to webmail then go to Personal Settings and the Identities tab. Make sure your email address entered there is correct. As well as being your "from" address this is also where replies go to.

[#3]

Thats ok , same as current mailbox in top right

[#4]

Sorry, I'd forgotten the Current Mailbox shown top right in webmail is the same as the default identity.
In your email address the domain following the @ will be username.plus.com where username is the one you use to login to the Member Centre and webmail. But is the local part before the @ the same as you chose for your first mailbox when you created it in Manage My Mail (http://email.plus.net) on Wednesday? For example is the mailbox name your first name, ie first_name@,  but in webmail it's username@?

[#5]

I've resolved this but am somewhat confused by the way webmail is working
I named the deafault account Primary so Primary@MyUserName.plus.com
I then created an E_Mail account which was Myusername@ so in full MyUserName@Myusername.plus.com
When I signed into to Webmail I signed in with just MyUserName  and everything that was displayed appearred to be the MyUserName@Myusername.plus.com account
but it seems it was actually the default account but anything sent from here was tagged as being sent from MyUserName@Myusername.plus.com account
But when I signed in with MyUserName + MyUsername then I could see all the E_Mails sent to the MyUserName@Myusername.plus.com account

[#6]

In webmail, as things stand you need to ignore what it says against Current Mailbox in the top right-hand corner. The value there is the default identity which will be used in the "From" field for sending messages (and "To" field in replies). The reason it's called Current Mailbox is because once the identity is set correctly that is what it will be.
Using your examples, when you login to webmail using just myusername the content of the default mailbox will be shown. This is the primary mailbox and the identity for that is primary@myusername.plus.com. The current setting is incorrect and to fix it you need to change the value (as described in reply #1) to primary@myusername.plus.com. Once that correction has been made new messages will be sent with that "From" address and replies will come back to the primary mailbox accessed by logging in using just myusername.
When you login to webmail using myusername+myusername the content of the myusername mailbox will be displayed, which as you said contains all messages sent to myusername@myusername.plus.com. For this mailbox I think the default identity is correct so there's no need to change it.
The explanation above is based on webmail, but the same settings and principles apply if an email client is being used. It's how the Plusnet email system works.