If you decide to go for new springs then I would suggest going directly to a spring manufacturer such as Faulkner Springs -
http://www.dfaulknersprings.com/That is where Demon Tweeks et al, get them from. So you might as well cut out the middle man and get exactly what you require.
You need to know the spring ID, spring rate and free length, so first you need to take the spring off the car and off the damper.
The spring ID - usually 2.25inch on a Midas, but do check as other sizes have been fitted by different builders.
The spring rate in lb in - the easy way to check this is with a bathroom scales a tape measure and a piece wood, or metal, that will allow you to push (or sit) on the end of the spring. Did I say easy way

well if you've got someone to help you take the measurement it is quite easy, but I've managed to do it in the past by sitting on a piece of wood, on top of the spring, on top of the scales and measuring the length of the spring before and after. So if you weigh 12 stones, that's 168lbs, and the spring deflects by about 1.7 inches that's a 100lb in spring. If the deflection is just over 2 inches its probably an 80lb spring, or if it's around 1.4 inches then its a 120lb spring. If you don't weigh 12 stones then you will have to do the maths for yourself. Actually there's benefit to being heavier here as the bigger the load, the bigger the spring deflection and the easier it is to get a good measurement. Don't worry about measuring too accurately though, the springs will be set to the nearest 10lb increment when new, but inevitably settle down and get a little stiffer with age. Faulkners don't list 90lb, or 110lb springs, but can supply them if you ask.
The free length - the length of the spring when it is standing free (off the damper) with no load on it. Again it may have settled down to being a bit shorter than the original length. You want your car to ride a bit lower and the rear spring acts pretty well directly on the wheel centreline (at least with the original Midas brackets) so one inch of spring height reduction will be one inch of ride height reduction. However, if you opt for a softer or stiffer spring than you had before then this will change the static ride height too. Faulkners list 12", 14" and 16" free lengths and if your required length falls between these then you should go for the shorter option and pack it up with spring seat packers, or a helper spring, or both (the helper spring makes sure that the spring doesn't rattle loose at full damper travel, so check that too).