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If you have only just had it installed I believe it can take up to 10 days to stablise.
Did you run the speed test on wireless or a wired connection, and did you have anything else streaming, downloading or other internet devices in use at the same time ? - It may be worth running the speed test from one of the other speed test servers too to see how it compares.
You may have a stuck profile, something you could check with +Net, but I think you have to contact SSE to ask. I am lead to believe fibre doesn't have a training period as adsl does, but I would still give it over the weekend to try and sort itself out, then get on the phone if need be.