Barbel Mick is right, brush up on the consumers rights act , which gives you 30 days to reject a defective/misrepresented bike if bought from a dealer for a full refund.
Be polite when speaking to them( getting angry will do you no good) ,but firm, show you know your rights.
Be prepared to walk away, leaving the bike with them,to either sort it or refund, before the 30 days are up.
Keep notes of all correspondence.
Good Luck