Did you not look over and test the bike when you collected it? Surely you would have seen many of the faults that you have detailed upon collection, including chain, reflectors and bell.
If the bike had been boxed and sent out, then it would have been up to you to carry out any final assembly and carry out a final pdi, including the fitting of any reflectors etc. These should have been supplied in a separate packet.
You state that the build quality is awful, yet rode the bike for 150 miles! That is rather confusing, as why would you ride a bike with awful build quality, rather than just rejecting it as not fit for purpose to begin with.
In any event I'm surprised that the bike was released if payment had not been received in full, be this discounted or not, which leaves an odd question mark over the discount. From the link that you supplied to the top cash back site, the agreement appears to be with the cash back company, not the dealer. No dealer is going to double discount.
Nobby Nic tyres aren't really designed for road use, but I'd certainly like to see photos of the disintegrated tyres, and what did you ride on for the remaining 110 miles?
The screw that you mention, might have been overtightened from factory and simply not spotted by the dealer, and if the bike had been sent out, rather than collected, then the same could be said of the torque settings, as you would again be responsible for the pdi. Also remember that Haibikes are renowned for suspension bolts working loose almost within seconds of being ridden, and many owners use cable ties to prevent them being lost.
As for the chain, that is hardly a big issue, and quite why you feel that Shimano is in your own words "shitty" is beyond me. It would very much be an over exaggeration to say that the bike would be unusable for two weeks, simply over the cost and fitting of a chain, be this Shimano, SRAM, KMC or whatever. If you can use a torque wrench to correctly tension bolts, then fitting a new chain is no big issue.
As I see it, you sound as though you have either changed your mind over the bike and are trying to back out of the deal, or you are simply narked because you didn't get one over the dealer on the purchase cost. There are two sides to every story, and I rather suspect that this one has another.
If you really do feel that you do have a grievance, then go to citizens advice and seek guidance, and also contact your credit company, as they might well be able to offer guidance. Don't expect an easy ride though, as comments and observations are going to be far harsher than mine.
You could also ask advice on the UK pedelcs forum, which would have more relevant advice in respect of UK consumer law.
http://www.pedelecs.co.uk/forum/
I also think that by joining the
http://www.cyclinguk.org/ that you get free legal advice, but any consumer advice centre should be help to help you
Good luck with things, and hopefully all be resolved amicably, as it would be a shame to let what seems like a few minor issues get in the way of what should be a pleasurable purchase.
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