* Margins fell in Rotterdam, Singapore, Med and U.S. Gulf * Only U.S. Gulf refiners cracking Brent saw improvement * Strong crude prices and weak end demand hurting refiners LONDON, Oct 5 (Reuters) - Oil refining margins broadly fell around the world last week, as a jump in crude oil prices which was not matched by product prices hit plant profits from Rotterdam to Singapore, according to Reuters data. Complex refiners suffered most from relatively strong crude oil prices last week -- which at times traded above $71 a barrel -- less than $4 a barrel off the high for the year so far in a time of seasonally weak end user demand. In Rotterdam, part of Europe's oil hub, complex refiners posted an average profit of $3.36 after processing one barrel of North Sea Brent last week, down by almost 40 cents from the previous week. Simple refiners around Rotterdam saw profits ease slightly to $3.41 a barrel from $3.42 a barrel. Simple margins around Europe were propped up by fuel oil as recent run cuts and the industry trend in past years to upgrade units to process the further refine the heavy product have cut supplies. Margins on heavier crude oils also fell for complex refiners. In the Mediterranean, complex margins on Russian Urals crude were $2.06 a barrel, down by 8 cents over the week. Simple Urals margins rose slightly to $2.01 a barrel. In Asia, simple margins on Dubai crude collapsed by almost $1 to just 29 cents a barrel. Complex margins fell to $2.04 from $3.65 a barrel. U.S. Gulf Coast refiners cracking Brent crude oil -- which was weaker than West Texas Intermediate (WTI) last week -- saw some gains. Hypothetical arbitrage Brent in the U.S. Gulf might have assumed a profit of $1.72 a barrel from $1.02 the previous week. Reuters data is a rough indicator of global margins and may not match with individual refineries' profit levels. (Reporting by David Sheppard; editing by James Jukwey)

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