Ian, I think your car is from the era when MMC bought some offshore low grade leaf springs & the issues you describe, other owners have had
This firm used to be (may now be back) the original OE manufacturer, using pukka quality steel & hand forged in the heartland of proper heavy engineering
Owen Springs, Rotherham, tel 01709 710700
Owen (I think) now supply MMC and probably aftermarket suppliers like Mulfab, but Caveat Emptor