Twelfth Night have just signed a record deal so that they can release definitive versions of their albums from their back catalog. Their best album is Live & Let Live - the live album of their last concerts with (now sadly deceased) Geoff Mann from 1983 - this contains the definitive version of Sequences (to Twelfth Night fans, what Grendel is to Marillion fans). Failing that, I would suggest you consider Collectors Item - a compilation of their best work. There are 2 versions of this album on CD - either is fine but, for my money, the one released by Cyclops is the better one. Both contain the 19 minute song - The Collector - recorded especially for the compilation with Geoff Mann - as it was written before he left (and performed a few times) although never recorded in studio.
Other good studio albums are Fact and Fiction & Art & Illusion. These (and most of their other albums) were (re)released by Cyclops with extra tracks. If you want to learn a bit more, though - try here: www.twelfthnight.info You can buy merchandise, including albums (many have now sold out though and are awaiting the definitve release) direct from their drummer, Brian Devoil (as I have done).
Hope that helps 
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