My first question: is it lawful to demand a penalty charge of £120 PLUS the original £9 ULEZ fee?
Yes it is, but it is
not lawful to demand the £9 toll on a PCN. This was decided quite some time ago in 2018, (by the Chief Adjudicator, no less), at the Traffic Penalty Tribunal, but as they don't offer an online searchable Statutory Register, it's difficult to see if there have been any decisions after this.
These regulations below define what can go into a PCN : -
https://www.legislation.gov.uk/uksi/2013/1783/regulation/7/madeThe first thing to tell you is that having receive the Charge Certificate, you have no rights of appeal any more. This doesn't mean 'game-over, cough-up', but you will have to wait for the Order for Recovery, issued when they register the debt at the Traffic Enforcement Centre. This last stage allows you to submit a Witness Statement to TEC that you didn't receive the PCN. OK, you did receive it, but well outside the period in which you could respond to it. That, for me, is non-receipt.
The alternative to this is to write to the council, explain the circumstances and ask them to cancel the Charge Certificate and accept your representations. This would be outside the legal process, so they are under no obligation to do anything. If they won't cancel the CC, then you just wait for the OfR. They cannot begin to recover the debt until this is done.
Once you are back to the PCN stage, you can submit reps or pay.
Second: Is there any way I can present the fact that I was away (which I can evidence) in an appeal, or will they just say that I should have had someone checking my mail for me?
Yes, of course you can, but they wouuld need to see some sort of collateral. Essentially this is an argumnt for being offered the discount, because you were away when the PCN discount period expired.
Third: The reason I was in Bradford in the first place was that I was performing in a band, unpaid, for a charity gig. It was also my first time driving in Bradford since the ULEZ came in and I was unaware that it existed. I didn't see any signs because I was concentrating on my driving. Are any of these facts worth including in my appeal?
You can say that you were unaware of the CAZ and saw no signs familiar to you. It is unlikely they will cancel the PCN though, although you might get the discount re-offered. BTW don't rely on any statements from council officials over the phone. Most of them are totally ignorant of the law they purport to enforce, relying on various IT systems that write letters for them.