Thanks guys, good advice...
Quick bit of info as to what the problem was - basically the domain expired. Our domain registrar had an incorrect email on file and auto renew was off - this is all sorted now, thanks for your patience.
Duma, your issue will most likely sort itself out over the coming days as a lot of this tends to be down to do caching and domain propagation. Looking at what's my DNS it seems to indicate that we've still got a few locations where it's yet to propagate and most registrars won't touch anything until at least 48hrs have passed (which it has), I'll keep monitoring this...
If anyone gets any issues then please let me or Simon know and we'll jump on them ASAP.
Cheers,