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* FTSE 100 up 0.1%, FTSE 250 down 0.2%
* U.S. tariffs on China to remain in place for now
* RBS slips on rating cut
* Diploma, Tullow lead midcaps lower
Jan 15 (Reuters) - London's main index eked out modest gains on Wednesday, hours before the imminent inking of an initial U.S.-China trade deal, though sentiment was tempered as Washington said tariffs on Beijing would not be rolled back immediately.
U.S. Treasury Secretary Steven Mnuchin said tariffs on Chinese goods would not be repealed until the completion of a Phase 2 agreement, planting some doubt in the minds of investors who were betting on a quick de-escalation of tensions.
As widely reported, the eventual removal of tariffs by Washington, which is not expected until after November's presidential elections, would depend on Beijing's compliance with the Phase 1 accord.
The FTSE 100 edged 0.1% higher. Royal Bank of Scotland lagged, however, giving up 3% after a rating downgrade by Barclays.
The FTSE 250 dipped 0.2%, weighed down by technical products and services provider Diploma, which skidded 3.4% after its trading update and a 2.4% drop in Tullow Oil after the company reported a $1.5 billion write-down. (Reporting by Shashwat Awasthi in Bengaluru, Editing by Sherry Jacob-Phillips)