RE: Sunday Times13 Jul 2018 14:50
Yes : There is trading after 4:30pm for 5 mins. In this period the computer system that runs the stock exchange tries to match up any outstanding orders to BUY on 'one side of the books' with orders to SELL on the other.
During that 5 minute period of time (and if you pay for access to this 'after normal hours service' )you can make a bid to buy (or offer to sell) but within a fixed range (5% ...so you couldn't make a silly offer!). The computer system tries to cross off as many buy/sell orders as possible so it looks for the biggest volumes to buy or sell first and works down the list. (I think of it as a crossing off period as it crosses off buys and sells, but the system refers to it as Uncrossing trade).
After 5 mins there is another 30 seconds and the market is closed and you see the closing price reported.
**** The aim is to come up with a closing price for the day and a starting price for the next day *****
The next day, there is a period just before the stock market opens to Joe Public at 8:00am (UK) whereby those who pay for the service have a small window of time before 8:00am to buy/sell shares...hence by the time the average punter has access to buy the share price has moved!