Hi, thanks for getting back to me.
Seems to be Random and not specific to any particular Numbers. My wife is on EE and it happens on and off with Texts I send to her, sometimes it even says its not Delivered but she's received it, other times its not delivered then she receives half an hour later.
Another odd thing is the Number I tried texting this morning 5 times the message box I is Blue then comes up Not Delivered next to it, the last time I sent it the message box is Green.
One other thing, someone sent me 4 messages at the end of January, I only received them on 12th February after I restarted my phone, so I restart it regularly now but it hasnt cured the problem I'm having now.
I can confirm that the SMS Centre Number is the same and which I checked after reading other posts on here and also WiFi calling is switched off as well, I checked both those after reading posts on here by people having Text Problems.