I've no idea what SQ's policy on seat assignment refunds is, but certainly BA has in its contract that it will not refund seat fees if the passenger chooses to cancel. That's not uncommon in the industry, so check the policy before you go down a line under which contract law will apply and you can't win.

What I don't understand is whether or not you accepted the revised flight time. If you did, then obviously you don't have a case. If you didn't then you'll be entitled to the refund, but that's something you're being offered anyway.

Aside from seemingly trying to be excessively litigious, I'm not sure what you hope to achieve given you pretty much already have the result you want hassle-free. There's a moment when a light bulb should come on and you accept the effort isn't worth it for a few dollars, particularly if your case isn't especially watertight.