Just one other point. If the electrician got the OP to accept the changes by stating "that's what the regs say" then he can't use customer acceptance of the changes as a get out. It sound very much like he's lied to the customer and you can't expect a non-qualified customer to know the details of all the regs.
True though this may one of those situations where there is not enough documented evidence either way. The myhammer site specifically tells the customer and supplier that they have entered into a binding agreement for what that is worth since the 10mm CSA SWA (or 4mm SWA) is apparently not documented.
I suspect the best the OP can hope for is
- Reasonable response from the electrician with some partial refund. I can not imagine the electrician replacing the cable unless under orders to so so (I may be wrong but seems a stretch)
- Some sense in the back of the electricians mind that he needs to avoid such problems in the future by being honest with customers - do they ever learn though? probably not.