Depends on whether you're in the north or the south :)
In the south, I believe it's [i], pronounced exactly like the letter "i".
In the north, I think it's a bit further back than "i" and closer to a German "ü", though not quite the same (it's not rounded). IPA (phonetic) symbol [ɨ]. Similar to Polish "y" or Russian "ы".
They explain the vowel sounds in the second part of this lesson, as well: http://downloads.bbc.co.uk/wales/catchphrase/catchphrase1/unit_1.mp3
Wiktionary spells it [sɨ̞t]. So I suppose you're right: lowered [ɨ] is pretty much the same as centralised [ɪ].
And the long version, e.g. in tu "side", Wiktionary spells /tɨː/.
In South Wales, the vowels are written as /ɪ/ and /iː/, pretty similar to the ones in "sit" and "seat" in English.
So in the South, i and u and (in a final syllable) y all have the same pronunciation, while in the north, u and final y are the same as each other but are a bit further back than i.
Well, the long version is as close as long /i:/, the short version apparently slightly laxed or slightly more open, similarly to /ɪ/.
So the main differences between the long and short North Wales version seems to be openness; the main differences between the South and North versions seems to be backness.