UK stock markets swung into positive territory on Thursday though gains were only modest as investors awaited further news on the Greek debt crisis, after creditors issued an ultimatum to Prime Minister Alexis Tsipras.As negotiations over the next tranche of bailout aid come down to the wire, lenders gave Greece's government until 1100 CET (1000 BST) on Thursday morning to submit a "workable" proposal.As many had predicted, Greece failed to deliver a new plan, prompting creditors themselves to send their own document to the Eurogroup which they hope could represent a basis for an agreement. Athens, however, has not signed off on the creditor-approved proposals and reportedly sent its own separate document to the Eurogroup.Officials must come to an agreement over the coming days or Greece risks defaulting on its debts, with a €1.6bn repayment to the IMF due on 30 June.Despite the uncertainty, the FTSE 100, which had dropped to a low of 6,813.72 early on, was trading up 0.16% at 6,855.78 by midday."The hour of reckoning is near, and traders are holding their breath as the deadline looms. Equity markets are quietly confident that an agreement will be reached in some shape or form," said analyst David Madden from IG.A raft of economic data from the States will also be in focus on Thursday afternoon: analysts predict a 6,000 rise in jobless claims, 0.5% and 0.7% increases in personal income and spending respectively, and a small rise in the Markit US services PMI.Admiral drops, Sage reboundsInsurer Admiral was trading sharply lower after Citigroup downgraded the stock to 'sell' from 'neutral', saying it's a high-quality name, but fully valued. The bank said it sees limited room for price-to-earnings expansion, given the growth profile is unlikely to return to historical levels, even assuming an upswing in UK motor rates.Shares in software company Sage pushed up as Investec upgraded the stock, following sharp losses in the previous session on the back of its Capital Markets Day. Investec raised the stock to 'hold' from 'sell'.Blue chips Compass, Experian and United Utilities were lower after going ex-dividend.Supermarket giant Tesco advanced on rumours that private equity firms Affinity Equity Partners, Carlyle Group and CVC Capital Partners were among seven or eight preliminary bidders for Tesco's South Korean unit.Recycled packing maker DS Smith gained after posting a 20% rise in full-year pre-tax profit and a 14% increase in the full-year dividend.Department store Debenhams also advanced after saying it was on track to meet full-year gross profit targets, helped by a 0.9% improvement in like-for-like sales.Market MoverstechMARK 3,205.95 +0.25%FTSE 100 6,855.78 +0.16%FTSE 250 17,971.18 +0.26%FTSE 100 - RisersSage Group (SGE) 530.50p +2.81%Mondi (MNDI) 1,444.00p +1.91%ARM Holdings (ARM) 1,146.00p +1.87%Barratt Developments (BDEV) 629.00p +1.78%Royal Bank of Scotland Group (RBS) 368.60p +1.65%Taylor Wimpey (TW.) 191.80p +1.27%ITV (ITV) 276.70p +1.24%Old Mutual (OML) 211.30p +1.00%Capita (CPI) 1,272.00p +0.87%BP (BP.) 445.90p +0.86%FTSE 100 - FallersUnited Utilities Group (UU.) 961.00p -3.08%Admiral Group (ADM) 1,409.00p -1.54%Compass Group (CPG) 1,097.00p -1.44%Weir Group (WEIR) 1,778.00p -1.22%BHP Billiton (BLT) 1,359.50p -1.02%Experian (EXPN) 1,198.00p -0.99%TUI AG Reg Shs (DI) (TUI) 1,151.00p -0.86%AstraZeneca (AZN) 4,219.50p -0.72%GKN (GKN) 349.80p -0.71%RSA Insurance Group (RSA) 414.20p -0.62%FTSE 250 - RisersPetrofac Ltd. (PFC) 976.00p +4.11%Bank of Georgia Holdings (BGEO) 1,944.00p +3.85%Smith (DS) (SMDS) 387.10p +2.95%Pets at Home Group (PETS) 301.60p +2.94%Capital & Counties Properties (CAPC) 434.20p +2.70%Telecom Plus (TEP) 904.50p +2.67%Berkeley Group Holdings (The) (BKG) 3,392.00p +2.60%Workspace Group (WKP) 940.50p +2.45%Unite Group (UTG) 597.00p +2.05%Hunting (HTG) 631.50p +2.02%FTSE 250 - FallersElectrocomponents (ECM) 222.00p -3.31%Jimmy Choo (CHOO) 155.50p -2.81%PayPoint (PAY) 991.50p -2.79%JD Sports Fashion (JD.) 683.50p -2.77%Genus (GNS) 1,429.00p -2.32%Foxtons Group (FOXT) 249.00p -2.28%Fidelity China Special Situations (FCSS) 160.70p -2.25%Cairn Energy (CNE) 176.00p -2.11%Mitie Group (MTO) 319.00p -2.09%Allied Minds (ALM) 547.00p -2.06%