I am looking for a free source of detailed stock data (bid and ask prices) with an interval of <1m. This is for testing out trading theories - therefore does not have to be live nor up to date. I would ideally like a source that could supply this data over a period of a day.
I know of and have used the Yahoo .csv 'API' for getting quotes; this offers a minimum interval of 1 day (as far as I can see) but I would like a source with an interval of <1 minute. For example, the Yahoo chart http://uk.finance.yahoo.com/echarts?s=BP.L#symbol=BP.L;range=1d actually has a very small interval between plotted prices; is there a way of leveraging this for example?