This is just my opinion.
Save yourself some money. rear springs for the saloon have almost no difference whatsoever. Think about it, petrol and diesel all have the same running gear and suspension parts at the rear. This is from the 1.8 petrol 2.0 and 2.5v6 to the 2.0 diesel.
The rear of the shell is the same weight so in reality the rear spring is a rear spring, end of.
There is one difference that I know of, later cars did away with the anti roll bar so the springs were a little stronger but not so you and I would notice perhaps a little less roll if you also have the ARB fitted and if they take a miniscule share on the minimal extra weight of a diesel lump from the front end then it's there to be used .
When I did my rear end rebuild 18 month ago on my 2.5 V6 I took advice from a very senior trader/member on here and purchased
these OEM springs for a fraction of the cost of other OEM springs.
They are advertised as diesel springs but as I say, the advice I had is from a technical guru.
The choice is yours but all I can tell you is my 2.5v6 launch car sits perfect and rides as it should.
Cheers