The quality of the rear springs Morgan fitted to my 2012 Plus 4 was dreadful too, worse than poor tempering, I would put money on them never being tampered at all! A pair of BCC asymmetric anti-tramp springs paired with a set of Bilstein dampers from SSL sorted it, to finish the rear end off a Panhard rod was added.
The first owner must have hated the original front end because within just 3.000 miles he had spent £6k on a Mulfab front wishbone conversion, this works extremely well and combined with my rear end improvements gives fantastic handling and a lovely compliant ride

I have no idea where Morgan were sourcing their springs from in 2011/12 but they were absolutely atrocious!