In reply to Stone_donkey:
I have a short back, so everything seems long on me... But thinking about items that are unusually long, I remember this:
http://www.blacks.co.uk/mens/109890-peter-storm-mens-merino-zip-base-layer-...
Yes, I know it's 'Peter Storm', but they were previously sold under Blacks' 'Technicals' range, just rebranded for some reason, and the merino is rather nice ('The Merino Company'; earlier incarnations used MAPP merino, as used by Arc'teryx...). £50 was inappropriate for the PS market sector, but they're reduced to £25, or £20 for the crew neck version.
Decathlon stuff is cut pretty slim, so probably worth a look. I like their Techwinter 500 thick base layers; an Arc'teryx feel at a Decathlon price. Wrote a review for their website last night...
The longest base layers? North Cape Rhovyl Therm, now long defunct...