UK stocks gains on Thursday with decent gains by insurers Aviva and Friends Life offsetting weakness in the mining sector, as investors took an optimistic approach ahead of a much-anticipated central bank decision in Europe.The European Central Bank (ECB) is widely expected to keep policy unchanged when it meets in Cyprus, but markets will be looking out for further details of its quantitative easing (QE) package first announced at its January meeting.A decision will be announced at 12:45 and president Mario Draghi will hold a press conference at 13:30."The Eurozone markets as a whole were posting big gains as lunch-time approached, with the expectation that Draghi's comments will only spur on further growth," said analyst Connor Campbell from Spreadex.London's FTSE 100 was up 0.4% at 6,947.54 by midday, as the Bank of England decided to leave interest rates unchanged, as expected. The Bank said at noon that the Bank Rate was left at 0.5%, while the asset purchase programme was maintained at £375bn.According to economists at the EY ITEM Club, the Monetary Policy Committee "will most likely wait until February 2016 to increase interest rates" with inflation likely to stay low in the near term.Aviva and Friends Life jump, miners fallBoth Aviva and Friends Life gained on Thursday after impressing the market with their 2014 results, ahead of their proposed merger to create the largest life and pensions group in the UK. Aviva said operating profits were up 6% at £2.2bn, while Friends Life's bottom line improved 38% to £556m.However, sector peer Admiral missed estimates with last year's numbers on the back of falling premiums as pre-tax profits declined 4% to £357m, while the full-year dividend was 1% lower.Mining stocks were mostly weaker after China lowered its forecast for economic growth this year to "around 7%", which would mark the lowest rate of expansion in 11 years. Rio Tinto, Fresnillo and Randgold Resources were all trading in the red.London Stock Exchange underwhelmed with a 19% rise in full-year adjusted profits to £491.7m, in line with forecasts. According to analysts at Numis, the shares are "well up with events", as it kept a 'hold' recommendation.Shares in Betfair surged after the online gaming group upgraded its guidance for full-year profits. Market MoverstechMARK 3,172.33 +0.39%FTSE 100 6,947.54 +0.41%FTSE 250 17,238.72 +0.60%FTSE 100 - RisersAviva (AV.) 564.50p +6.11%Friends Life Group Limited (FLG) 429.10p +5.64%ITV (ITV) 239.90p +2.30%Intertek Group (ITRK) 2,586.00p +2.29%Rolls-Royce Holdings (RR.) 971.50p +2.26%GKN (GKN) 363.40p +2.25%Standard Life (SL.) 431.90p +2.06%Schroders (SDR) 3,096.00p +2.04%Glencore (GLEN) 288.75p +1.94%Meggitt (MGGT) 553.50p +1.65%FTSE 100 - FallersRio Tinto (RIO) 3,002.50p -2.31%HSBC Holdings (HSBA) 573.00p -2.24%Standard Chartered (STAN) 1,010.50p -1.32%RSA Insurance Group (RSA) 421.20p -1.27%Aggreko (AGK) 1,618.00p -1.10%easyJet (EZJ) 1,719.00p -1.09%International Consolidated Airlines Group SA (CDI) (IAG) 563.50p -1.05%CRH (CRH) 1,774.00p -0.78%Ashtead Group (AHT) 1,137.00p -0.61%Shire Plc (SHP) 5,220.00p -0.57%FTSE 250 - RisersBetfair Group (BET) 2,073.00p +16.07%Spirax-Sarco Engineering (SPX) 3,331.00p +8.15%Hunting (HTG) 504.50p +6.52%Pace (PIC) 380.70p +5.87%Premier Farnell (PFL) 186.50p +3.84%Morgan Advanced Materials (MGAM) 334.10p +3.40%Home Retail Group (HOME) 202.50p +3.32%Premier Oil (PMO) 155.20p +3.26%Bodycote (BOY) 784.50p +3.22%Ophir Energy (OPHR) 139.80p +3.17%FTSE 250 - FallersAllied Minds (ALM) 564.00p -9.54%Afren (AFR) 6.18p -6.36%Cobham (COB) 324.80p -2.46%Domino Printing Sciences (DNO) 731.00p -2.14%Lonmin (LMI) 139.20p -2.04%Renishaw (RSW) 2,506.00p -1.99%Euromoney Institutional Investor (ERM) 1,039.00p -1.89%Domino's Pizza Group (DOM) 738.00p -1.80%Aveva Group (AVV) 1,503.00p -1.70%Infinis Energy (INFI) 193.00p -1.58%