I can't help you with the PCNs but I use this bridge a fair bit. I myself have forgotten to pay numerous times, and ended up paying a day late and didn't get PCNs - worth knowing as their site says do not pay late as we cannot tally up your payment with the crossing. Get the app. The app is great, I have it now and it automatically deducts the money immediately after I have crossed so I don't need to worry. The fee is £2, its a shame you didn't have the funds to pay, as it's now turned into 70 times that.