By Stefano Rebaudo (Reuters) -Sterling gave up some gains but remained in positive territory against a slightly weakening dollar on Monday while optimism about a diplomatic solution to the Russian-Ukraine standoff faded, dampening risk appetite. The Kremlin said there were no concrete plans for a summit over Ukraine between Russian President Vladimir Putin and his U.S. counterpart Joe Biden, after the French president said the two leaders had agreed on a meeting in principle. The pound was up 0.15% against the dollar at $1.3608, not far off its highest level since Jan. 20 at $1.3643. It was up…