Nice looking car and I love the colour, very period.
The camber on the rear wheels looks to be down to the drive shafts. Late Mk IV Spitfires (from '72?) and all 1500's used longer drive shafts than Heralds, Vitesses and early Spitfires to introduce negative camber and improve the handling.
You can tell which type you have by looking at the tie bars - if their ends are at 90 degrees to the shaft you'll have short shafts, if they're angled slightly you'll have long ones (see discussions in the links below). The body brackets are also slightly different. From the look of the back of your car I'd say you have the long ones.
To change to short shafts you'll need: a pair of shafts from an early Spit, Herald or Vitesse; a pair of straight tie rods; and a pair of early body mounting brackets to attach the tie rods to the body. That lot will just bolt up to your existing spring and reduce the amount of negative camber. You should also be able to sell your long shafts, tie rods and brackets for similar money to what you pay for the short ones.
If you already have the short shafts fitted then either you've got a ton of lead in the body at the back or your leaf spring is worn out. They're cheap enough to replace though -
http://www.ebay.co.uk/itm/REAR-LEAF-...IAAOSwExJXofAr
http://clubtriumph.eu/cgi-bin/forum1...?m-1308488838/
http://auskellian.com/paul/links_fil...hancements.htm