♦ The first time _ I HAVE tried sourdough pizza HAS BEEN -- 2) when I WAS in London about a month ago." 1) HAVE TRIED, HAS BEEN = they refer to a concluded event( trying pizza is a single moment). But you might also say: --- The first time _ I TRIED sourdough pizza WAS when I WAS in London about a month ago." -- using Past Perfect. But it's just a "month ago" so Present Perfect (HAS BEEN) is more suitable. 2) when I WAS in London = here "was" is Imperfective, a sort of continuous tense(--staying in a city is a long event--) which is used for setting a story in the Past, like in: ---"Once upon a time, there WAS a prince and a princess ..." but you could also say: "--- when I HAVE BEEN in London last month" because now you're back from London, so you are speaking about a concluded event.

You might express the same idea: " I first tried sourdough pizza about a month ago, when I was in London."

It's not quite right. You need to use past simple (when I was in London) because you use a past time (a month ago). If you use a past time expression you cannot use present perfect. I hope that helps :)