Perhaps point the shop owner to this thread, he can even converse with experts here who had built many hundreds if not thousands of wheels. At the very least he could see how his reputation is being damaged by his and his staff's bullshoot.
E-mail then, drawing on thread content as necessary to Cannondale, copy to Northfield Cycles. Describe what's happened and what you have asked NC for and what they've done; and that it's not satisfactory. What do you seek: £25 refund for the spoke replacement and trueing the first time round, replacement rear wheel (at least) which is 'fit for purpose' (ie doesn't 'fall apart' after 600 miles) and gratuitous transfer of your tyre and cassette onto that wheel.
Hopefully that Cannondale pressure will make Northfield Cycles deal with you fairly.
https://www.cannondale.com/en/Great Britain/contact-us-form
"Your local bike shop should be able to answer most questions about your bike – often more quickly than we can here at headquarters!
For all these reasons, we suggest you visit your local bike shop before contacting us at Cannondale directly. You’ll receive great service and support your local bicycle community at the same time! "
Local Authorized Cannondale Retailers
http://northfieldcycles.co.uk/
Cannondale said:
"I spent over 20 years in a store as a manager / mechanic and spokes coming loose and the odd spoke snapping can happen but if this happened on many occasions I would have normally contacted the warranty department just to see what their line on the matter was just so that I was aware if they could help or not. There is a product on the market called spoke freeze that I used to use when coming across this issue which is applied to the spoke thread during the re-truing / building process ( similar to Loctite ) which should hold the spoke in place.
I would try to speak to the store again."
[Cannondale gave] the response I posted above.