** HSBC shares in London see spike in tradingactivity with more than $400 mln traded in the stock by middaymaking it the heaviest traded across Europe
** Stock is most active on the FTSE 100 by somedistance with 1.5 the average monthly volumes traded
** Traders point to 20 mln shares that were crossed in asingle transaction around 0849 GMT. Order data shows another 20mln shares were traded in 4 blocks of 5 mln shares each soonafter
** Little price impact on stock which is down 0.4 pctlargely in line with the FTSE 100
** HSBC shares have seen volume spikes on 3 of the last 4Fridays (RM: sudip.kargupta.thomsonreuters.com@reuters.net; RM:alasdair.pal.thomsonreuters.com@reuters.net)
HSBC Holdings (HSBA)