CMC Markets actually publish a spread chart in real-time for each instrument, so you can check them for yourself. To check spreads simply select an instrument and you can put up a spread chart for that instrument (select spread charts from charting menu on individual products) - this will give you CMC's real time spreads throughout the day real time.
Interestingly, CMC seems to be focusing on improving the platform and making the spreads consistent across all market conditions as opposed to offering the narrowing spreads possible. With their new NextGen platform CMC vouch that they will be filling every ticket and spreads are consistent (they are good for example up to £200 per point on the UK 100 index.
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