Ready for my “just in case” stock as it was commented on the last mot (but not an advisory) that the front springs were getting heavy rust spots. This could cause stress risers which makes the spring vulnerable to breaking of you hit something like a pot hole – especially on cold winter days ….. which are approaching.
So might decide to fit them anyway as the whole original strut is looking a bit rough around the edges. Would make a good winter project.
It is always recommended to replace these springs in pairs to ensure the spring rate on each side is the same.
This is now on my list for this winter projects.
MG ZT info mods and maintenance 
Worth fitting your new springs. My front passenger spring failed suddenly at 71k after hitting a pothole, luckily at very low speed close to the house. On the A30 it would have probably cost me the tyre. I decided to fit new shocks and to de-rust and paint the strut mounts all in one go. Good luck.
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