Don't waste your time, or the opportunity, in trying to argue s14(2) fitness for purpose as it's likely to fail at the first hurdle - you have been riding it, therefore you have shown by your own actions that it is fit for the purpose as a means of transport. You'd fair better with a s14(3) satisfactory quality argument, citing the various faults and deficiencies that would not have arisen had the machine been of satisfactory quality. Leave aside the repair and/or servicing issues as a separate line of argument/negotitation. Arguing that the original machine is clearly of poor quality is your best chance of getting the manufacturer to roll over. Suggesting that the dealer displayed incompetence in trying to put right the original faults adds to that argument. Bringing into play the dealer's faults just confuses the issue so let that rest as a Plan B at the outset, using it when/if the first pitch fails.