Thanks Mitchell, I have actually managed to connect properly today and send my records through. I don't know what is different, apart from a little time perhaps to clear things out.
Here is a little more info if it is helpful:
The day before the fieldwork I had been testing while connected via WiFi. I was signed in the whole time, although most of the time when I went to send the data through to AGOL I would get an 'invalid taken' error and I would have to sign out, then sign back in, and the send would go through successfully every time. I assumed that this was something to do with a timeout but don't really know any better.
I would have been left signed in to AGOL on the Survey123 app on the iPad when it was taken offline and used in the field. Later that day after the fieldwork was completed I could not send the data once I was reconnected to our WiFi, and kept getting the 'q parameter' error. Signing out and back in had no effect.
The following morning I tried again, and it worked first time.
Cheers
Keith