Whilst I've not had any dealings (personally) with Athena/Lidl. The sequence of events seems to be fairly standard across all Private Parking Companies (PPC's) especially where ANPR systems are involved.
1. Parking Charge Notice.
2. Parking Charge Notice (Reminder).
3. Parking Charge Notice (Final Demand).
4. Debt Collection Agency Letter. Which will usually add another £40 or £50 on top of the original invoice.
5. Debt Collection Agency (Letter #2). Why haven't you paid? You have to pay! We'll tell our clients about you if you don't!
6. Debt Collection Agency (Letter #3). We don't like you anymore, your not giving us all your money. We're going to tell our clients to start county court proceedings. You're horrible and we hate you.
7. Debt Collection Agency (Letter #4). Look, ok, we've had a bit of a think about it, and we're going to give you one last chance to pay us, and as an incentive, we're going to knock a few quid off what we're asking you for. Aren't we wonderful.
It is important to note that the Debt Collectors cannot do anything to you, apart from writing you increasingly desperate letters using capital letters and lots of red ink.
As you've ignored it so far (which isn't the best advice anymore (but that's by-the-by)) my advice would be to carry on ignoring them. The only time that you need to take any further action is IF (and that's a big if) Athena (not their pet debt collectors) send you a Letter Before Action (or Letter Before Claim, depending on how they word it). Which would mean that they're planning on taking you to County Court. And even that's not a big deal with a properly worded defence, which we'll help you with if it gets to that.
After the debt collection letters get no response, they *might* just give up and chase easier targets that just roll over and pay up without question.