HMRC's response to RTI filing seems to have gone down again, over the last hour. Our phones have gone red hot with callers saying that they file FPS and don't get a response back. I've just tried one, and yes, it goes into the "pending" state where HMRC has received it but not yet responded with success or failure.
Over the last 6 weeks this has been happening more and more, as if HMRC's servers struggle to cope as the data volume they're dealing with increases. Presumably the big employers are coming live now as they were delayed initially.