Sometimes I would send a text and the other person would say they didn't receive it until hours later when they reply back, and this has happened with me being on the receiving end as well. I'm wondering what the tech reasons are for this, apart from the obvious like phones being switched off or not being in range of a signal.

Network issues? Congestion? Could be a number of reasons.

Can't remember that ever happening on the Three network, been with them for 4-5 years.........

I'd say it was all down to network though, nothing to do with the phone, especially seeing as it's other people your sending/recieving to that also have the problem