I ordered a special meal but the FA told me I'm not appearing as having any special meal, pointing to some hand-scribbled note with some seat numbers that didn't include mine..

I took some regular meal instead, and asked the FA to check whether it was a mistake in how I order the meal, or that it wasn't loaded for some reason. She promised the check and get back to me. Around 3 hours later, no word from the FA. I asked another FA and it turns out that my special meal appeared in the manifest and was loaded on the plane - it was some human error of the first FA (and maybe others as well).

I asked for a complaint form, but the FA tried to persuade me not to file a complaint and asked how she can make it up to me, offered me a "bottle of something" or a "wash kit from first class". I politely declined. The purser later came up and offered me 2,000 avios (ridiculous, I know). I said the minimum I would even consider as decent compensation is 20,000. The purser then said she can give me 10,000 and if I want anything beyond that I would need to contact customer support. I agreed to take the 10,000 for a start, and entered my email on her ipad for it to presumably auto-post to my account.

Now I'm wondering - what is the chance of getting a significant additional compensation and how to best go about getting it?