In a similar situation to you OP, only I bought a moped not a car and it was 18 months ago.
You should have sent your letter by registered post, it's only around €5 and now the garage can deny they ever got your letter. You'll know next time.
I gave 28 days notice and no reply so I paid €9 and lodged a claim with the Small Claims Court. The court register passed my claim to them and they have 15 days to respond or I win the case. Half that period has gone already, so maybe there is justice for the little guy in this country.
I sued for the full amount as ownership never passed to me. I doubt I'll get my money back but I hope to at least get all the money I spent on calls and registered letters.
Note, unless your car is €2,000, you won't be able to use the small claims court.