Receiving text messages late iPhone 4 - ios 7.1

So recently (in the last couple of days) my iPhone 4 has taken to receiving text messages around 30 mins late. The signal on my phone is consistently around 5 bars and there are no problems with my O2 service or indeed problems in the area with signal.

The only person I text really is my girlfriend who does not have an iPhone, so I deduce that this isn't a problem with iMessage

I've tried turning my phone off/on again, and there has been no difference (I have not tried restoring my phone as of yet, as I would rather do that as a last resort)

Data roaming and 3G are always turned off on my phone because I rather use wifi - however there has been a problem with wifi recently, but text messages shouldn't be affected by this surely....

Thanks in advance