As it turned out, the solution was quite simple – simple enough that it probably should have taken O2 less than the 5 days that it took to fix, but how and ever, all good now.
The solution: If you’ve previously been on O2, and you port away to another network, and then port back to O2 sometime later, if you were a Speakeasy pay-as-you-go customer you will automatically be ported back as a Speakeasy customer – even if you’re actually signing back on as a bill pay customer.
So, having been Speakeasy previously, and now porting back as a bill customer, I was trying to send texts, but the O2 system thought I was still Speakeasy – but had no credit – and therefore was preventing texts. This was only on their SMSC though, since I was able to make phone calls without a problem. (As it turns out, it must also have impacted on their wap service too, because I can now access the internet now also).
However, credit to O2 customer service in Limerick – very helpful!!!