Mid-caps plunged on Monday, with gains made by miners overshadowed by weakness in the travel and financial sectors, as Greece closed its banks and called for a public vote on bailout proposals by its creditors.As of 15:00, the FTSE 250 was trading 1.2% lower at 17,607.Travel tour operator Thomas Cook dropped sharply as investors expressed concerns about Greece and the impact of potential social unrest on the travel market, while sentiment was dampened further by the recent terrorist attack at a hotel in Tunisia.Financials Henderson and Man Group were trading in the red as market volatility increased across Europe.Gold miner Centamin was making gains as investors sought out safe-haven investments, namely gold.Ocado was boosted by reports in the Daily Telegraph which indicated that the firm is close to securing a deal with an international retailer, which would allow the company to expand outside the UK for the first time.OneSavings Bank extended Friday's gains, which came as Investec gave the stock a 'buy' rating and a price target of 365p in initial coverage. It forecast 28% loan growth or £1.1bn in 2015, marginally ahead of guidance, in a note to clients. RisersGrainger (GRI) 229.40p +2.64%Ocado Group (OCDO) 432.00p +1.48%Centamin (DI) (CEY) 63.25p +1.12%Serco Group (SRP) 121.00p +0.83%Daejan Holdings (DJAN) 5,720.00p +0.79%Acacia Mining (ACA) 309.00p +0.55%OneSavings Bank (OSB) 317.00p +0.54%COLT Group SA (COLT) 189.50p +0.53%Caledonia Investments (CLDN) 2,425.00p +0.41%Supergroup (SGP) 1,217.00p +0.41%FallersEuromoney Institutional Investor (ERM) 1,138.00p -5.72%Henderson Group (HGG) 262.70p -4.61%Bodycote (BOY) 688.00p -4.04%Thomas Cook Group (TCG) 138.70p -3.81%Man Group (EMG) 159.40p -3.34%Drax Group (DRX) 353.80p -3.33%Evraz (EVR) 123.50p -3.29%Go-Ahead Group (GOG) 2,610.00p -3.26%Allied Minds (ALM) 521.00p -3.25%Spire Healthcare Group (SPI) 328.40p -3.13%