London midday: Stocks fall on reports of Russian trucks crossing border

- Russian trucks enter Ukraine without persimmon- Investors cautious ahead of Jackson Hole- Petrofac chair steps down, LSE falls- Vodafone gains on AT&T bid hopestechMARK 2,815.11 -0.17%FTSE 100 6,760.20 -0.26%FTSE 250 15,840.73 +0.01%After a subdued start, UK stocks dropped into the red by Friday lunchtime on reports of trucks from the Russian aid convoy entering Ukraine without permission or heading towards checkpoints in the direction of Donetsk and Luhansk.Ukrainian state security chief Valentyn Nalivaychenko has been quoted as saying that the move is a "direct invasion by Russia of Ukraine". It had been agreed that the convoy could cross the border accompanied by the Red Cross, but it chose not to escort the vehicles because of the "volatile security situation". The FTSE 100, which had started on the back foot on nervousness ahead of a speech from Federal Reserve chair Janet Yellen, was trading 0.3% lower at 6,760 by midday.A three-day weekend was also likely behind the subdued mood, as investors refrain from taking on too much risk ahead of the Bank Holiday.Jackson HoleGeopolitics aside, on an otherwise quiet day for economic data investors were focusing on the Jackson Hole symposium of central bankers and keynote speech from Yellen as markets continue to speculate over the timing of the first rise in interest rates.A dovish view - stressing the need to keep borrowing costs low for now - could drive gains on equity markets with policymakers happy to await a stronger recovery before tightening, probably by the middle of next year. However, a hawkish standpoint could trigger a sell-off as investors bring forward their expectations for the first rate hike and take profits after the recent rally.Analysts at Charles Stanley said: "We conclude that the stock market is betting on a very dovish speech from Mrs Yellen. Given the title of this year's conference ('Re-Evaluating Labour Market Dynamics'), there exists plenty of scope for a dovish assessment."Meanwhile, European Central Bank (ECB) president Mario Draghi is also a guest speaker at Jackson Hole and will be closely watched by the market given recent calls for the ECB to act to boost growth in the Eurozone.Petrofac and LSE fallOilfield services group Petrofac was trading lower after its chairman of three years, Norman Murray, announced his resignation from the board. The company said that he will step down with immediate effect for "compassionate reasons".The London Stock Exchange reported a boost to profits from a surge in flotations in its first quarter but shares fell as it unveiled details of a £938m rights issue to bankroll its proposed acquisition of US financial index business Frank Russell.Telecoms group Vodafone was extending gains made on Thursday on the back of rumours that advisers of US giant AT&T are working on a cash bid worth more than 300p a share, compared with current prices of just over 200p. AT&T ruled out an offer back in January, meaning that it is now free to revisit an approach under UK takeover 'cooling off' rules.Banking stocks were in demand but had pared earlier gains. Lloyds and HSBC were both in positive territory but off their daily highs.FTSE 100 - RisersPearson (PSON) 1,141.00p +1.42%AstraZeneca (AZN) 4,445.00p +0.76%Babcock International Group (BAB) 1,125.00p +0.63%Randgold Resources Ltd. (RRS) 4,933.00p +0.59%Reckitt Benckiser Group (RB.) 5,235.00p +0.58%Vodafone Group (VOD) 203.70p +0.54%Tesco (TSCO) 246.80p +0.53%Lloyds Banking Group (LLOY) 75.63p +0.52%HSBC Holdings (HSBA) 644.20p +0.51%Hargreaves Lansdown (HL.) 1,138.00p +0.44%FTSE 100 - FallersSSE (SSE) 1,496.00p -1.58%Petrofac Ltd. (PFC) 1,110.00p -1.42%Rolls-Royce Holdings (RR.) 1,024.00p -1.25%BHP Billiton (BLT) 1,947.00p -1.22%CRH (CRH) 1,384.00p -1.21%Kingfisher (KGF) 303.80p -0.98%GlaxoSmithKline (GSK) 1,421.00p -0.94%Smith & Nephew (SN.) 1,054.00p -0.94%Capita (CPI) 1,206.00p -0.90%Shire Plc (SHP) 4,861.00p -0.88%FTSE 250 - RisersEssentra (ESNT) 858.00p +3.62%Fidessa Group (FDSA) 2,316.00p +2.93%Lonmin (LMI) 219.00p +2.87%Premier Oil (PMO) 342.70p +2.39%ITE Group (ITE) 198.80p +1.90%Kazakhmys (KAZ) 296.50p +1.85%Premier Farnell (PFL) 195.20p +1.67%Rank Group (RNK) 163.00p +1.62%Thomas Cook Group (TCG) 122.90p +1.57%Rentokil Initial (RTO) 126.40p +1.53%FTSE 250 - FallersIG Group Holdings (IGG) 597.50p -3.40%Supergroup (SGP) 1,101.00p -2.22%JD Sports Fashion (JD.) 390.10p -2.18%Oxford Instruments (OXIG) 1,183.00p -2.07%Computacenter (CCC) 615.00p -2.07%Barr (A.G.) (BAG) 633.50p -2.01%Halfords Group (HFD) 485.90p -1.98%Hays (HAS) 126.60p -1.86%Senior (SNR) 287.10p -1.81%Vedanta Resources (VED) 1,010.00p -1.66%BC