"frequent a restaurant" bedeutet, es regelmäßig aufzusuchen.
Ich würde sagen, eine passende formale Redewendung wäre:
"I dined at your restaurant", "visit" ist aber auch wohl sehr gebräuchlich.

You can't frequent something last week. "visited" is fine.

Neither the food I was served nor the service I experienced met my expectations.

Bist Du Dir im klaren darüber, wie 'German' Deine Kritik 'rüberkommt' ?!

There's nothing wrong with the rest, #3.

How do our British fellow-men see this ?

I support #1, but I don't much like "visit" here.
I support #3, unless this is supposed to sound very formal and stuffy.

What's wrong with visit?

Thanks for your help! I really appreciate it.
:-)

Perhaps this is partly a cultural thing, but it seems relatively unlikely that anyone would write a letter or an e-mail about a disappointing meal at a restaurant a week after the fact. The normal thing to do would be to ask to speak to the manager while you're still there, thus giving them a chance to correct whatever it is that you're so dissatisfied with. If they didn't or couldn't, you just wouldn't go back.

But to address the question about the verbs: 'go to' a restaurant is casual, 'eat at' is neutral, 'dine at' is formal.

'Visit' sounds less idiomatic, more Denglish -- not wrong exactly, but a little odd. Someone in restaurant management might talk about customers' repeat visits.

'Frequent' is so formal that it may sound pretentious, and it means to go to the same place regularly, as a habit, so it can't be used for only one visit.

'Attend' is wrong for places where people come and go all the time, like restaurants. It's used with performances, meetings, classes, etc. -- events where everyone needs to be there at the same time, and someone needs to know how many people are present.

hm is right, it's curiously difficult to find a verb that really fits this context I find.
How about
"I was a guest at your restaurant last week and I am writing to express my strong/extreme dissatisfaction with both the food I was served and the service I experienced."

Shorter too. :-)

Re #9 - there's nothing Denglish about visiting a restaurant. It's also perfectly idiomatic English:

http://www.buzztimebusiness.com/smarts/8-idea...
Customers who've never visited your restaurant probably wonder what it looks like inside.

http://www.latimes.com/food/dailydish/la-dd-n...
When you sit down at a table at the newly opened Brand 158 restaurant in Glendale, the host politely asks if you've visited the restaurant before.

http://www.nola.com/dining-guide/index.ssf/20...
I ended up visiting the restaurant three more times before writing my review...

Re ## 1 and 2 and 11.

Though (as other posts have demonstrated) it's not the only way to say it, "visited" is just fine--it's very normal, perfectly acceptable English. I agree with maxxpf and Kai here.

Kann sich der Wirt nach einer Woche wirklich noch an jeden Gast und jede Bestellung erinnern?

Ich halte einen solchen Brief für unsinnig. Wenn man in einem Lokal etwas an der Qualität der Speisen auszusetzen hat, dann sagt man das an Ort und Stelle der Bedienung bzw. dem Wirt. Und lässt das Essen zurückgehen.

In aller Ruhe die bemängelten Speisen aufzuessen, zu zahlen und dann erst eine Woche später einen Beschwerdebrief zu schreiben ist meiner Meinung nach ein sehr seltsames Verhalten. Einen solchen Gast wird kein Wirt ernstnehmen.

Dear all, I know how odd such a LETTER OF COMPLAINT must appear to you, but - believe it or not - this text type is one of the requirement for the Austrian finals (MATURA / ABITUR) and therefore teachers (students alike) have to put up with it.

Of course the (virtual) customers complain about inadequacies in the restaurant immediately, but in case the staff does not appoligize or react in a polite way, this is the justification for complaining to the management in writing - hence this text type is (still) part of the schedule, even if it may not be an adequate way of reacting any longer.

Anyway, thanks for all your opinions, explanations and practical advice! :-)

You attend a meeting but not a restaurant.

Wie wäre es mit:

I was very disappointed about the food and the service at your restaurant when I dined there last week.

Dann würde ich aber auch noch etwas genauer ausführen, weswegen Dir alles nicht gefiel, denn der Restaurantbesitzer muss ja eine Möglichkeit haben, Dir zu antworten und ggf. Dinge zu verbessern, sonst bringt Deine ganze Beschwerde doch nichts und Du wirst auch keine Rückantwort erhalten.