- BHP Billiton, Aberdeen, Babcock, GKN provide a lift- Weir falls on Metso merger speculation- Markets await global manufacturing PMIstechMARK 2,779.21 +0.53%FTSE 100 6,621.97 +0.36%FTSE 250 16,339.39 +0.40%UK stocks opened with small gains on Tuesday ahead of a data-heavy session with manufacturing indicators due out from across the globe.Mining giant BHP Billiton, asset manager Aberdeen and engineering groups Babcock and GKN were among the best performers in early trading. Weir, however, was lower on speculation that it could be eyeing up Finnish counterpart Metso.The FTSE 100 was up 0.4% at 6,622 in the opening hour in London.Sentiment was helped by a strong performance by US markets on Monday evening after Federal Reserve Chair Janet Yellen pledged continued "extraordinary support" for the US economy. She said there is "considerable slack" in the labour market and that the Fed would continue to support the recovery "for some time to come".Investors were digesting factory data from China overnight. The government's official manufacturing purchasing managers' index (PMI) increased from 50.2 to 50.3 in March; while HSBC's own manufacturing PMI slipped from 48.1 to 48, more or less in line with market forecasts.Manufacturing PMIs from the UK, Europe and US will also be released during today's session, along with the unemployment rate in the Eurozone.BHP gains on de-merger speculationBHP Billiton, the world's largest mining group, responded to market speculation about a potential de-merger of non-core operations, saying that a "simplification of our portfolio is a priority". Shares rose after the company did not dismiss recent reports that it is considering a $20bn spin-off of its less-profitable assets of aluminium, manganese, thermal coal and nickel.Aberdeen Asset Management rose despite saying that assets under management declined over the first two months of the year due to weakness in emerging markets. Babcock rose after it was named preferred bidder on a 21-year contract with the London Fire Brigade. GKN was also higher after buying Williams Hybrid Power which makes flywheel-based energy storage systems for the bus, truck and tram markets.Scottish engineer Weir, which works in the mining, oil and gas industries, was lower on reports that it could be on the verge of a blockbuster £8.5bn merger with Finnish rival Metso. It is thought that Weir could be prepared to pay as much as €30 per share for the company, giving it a market value of over €4bn (£3.3bn).Interdealer broker ICAP gained after saying overall results for the year are expected to be in line with current market consensus forecasts, despite the weak global economy, regulatory reforms and low interest rates. Jupiter Fund Management was in demand after agreeing to dispose of its Private Client & Charities operations, which represent 7% of its £31.7bn assets under management, to Rathbone Brothers.Consumer staples stocks were among the worst performers this morning with beverage groups Diageo and SABMiller registering losses, along with Coca-Cola bottling firm Coca-Cola HBC and householder products businesses Unilever and Reckitt Benckiser.FTSE 100 - RisersAberdeen Asset Management (ADN) 411.70p +5.48%Babcock International Group (BAB) 1,388.00p +3.04%BHP Billiton (BLT) 1,894.50p +2.74%ARM Holdings (ARM) 1,021.00p +2.30%Aviva (AV.) 484.10p +1.49%Rolls-Royce Holdings (RR.) 1,089.00p +1.40%GKN (GKN) 395.40p +1.31%CRH (CRH) 1,696.00p +1.25%Legal & General Group (LGEN) 207.20p +1.22%easyJet (EZJ) 1,735.00p +1.17%FTSE 100 - FallersPearson (PSON) 1,036.00p -2.54%Weir Group (WEIR) 2,496.00p -1.58%Sainsbury (J) (SBRY) 312.10p -1.27%Unilever (ULVR) 2,540.00p -0.82%Tesco (TSCO) 293.05p -0.80%Diageo (DGE) 1,847.00p -0.75%Coca-Cola HBC AG (CDI) (CCH) 1,484.00p -0.67%Reckitt Benckiser Group (RB.) 4,858.00p -0.59%British American Tobacco (BATS) 3,317.00p -0.55%Burberry Group (BRBY) 1,391.00p -0.29%FTSE 250 - RisersPayPoint (PAY) 1,199.00p +4.72%Go-Ahead Group (GOG) 1,931.00p +3.65%Jupiter Fund Management (JUP) 412.80p +2.99%Essentra (ESNT) 897.00p +2.81%ICAP (IAP) 388.10p +2.78%JD Sports Fashion (JD.) 1,639.00p +2.44%Kentz Corporation Ltd. (KENZ) 770.50p +2.26%Greencore Group (GNC) 281.40p +2.07%Entertainment One Limited (ETO) 336.70p +2.03%Dechra Pharmaceuticals (DPH) 681.50p +2.02%FTSE 250 - FallersDunelm Group (DNLM) 920.50p -2.64%FirstGroup (FGP) 143.00p -1.99%NMC Health (NMC) 496.70p -1.55%Catlin Group Ltd. (CGL) 530.50p -1.39%EnQuest (ENQ) 122.50p -1.21%CSR (CSR) 717.00p -1.17%St. Modwen Properties (SMP) 395.40p -1.15%Ocado Group (OCDO) 455.70p -1.06%Fidessa Group (FDSA) 2,512.00p -0.99%Essar Energy (ESSR) 67.20p -0.89%BC