So a complex question, in short a rear tire change (of both tires) will not affect alignment.
In detail going through you post I can make the following notes but not a determination.
1. A front tire change of just one side can affect the alignment depending on level of wear of the tire on the other side.
2. A rear tire change will not affect the alignment as long as they don't touch the rear alignment (which they typically wouldn't).
3. New tires can be defective, especially lower tier tires. If one of the tires has a significant enough variation in circumference the alignment will be affected.
4. Check tire pressure, high difference of pressure will affect alignment.
5. It might be perception, you might simply had the alignment going of because of suspension wear but did not consciously check it until tires were changed.
Thanks VII7 for taking the trouble to respond.
I agree, fitting rear tyres won't affect steering wheel alignment, however it has!
1) There was no issue after fitting single front tyre (I drove over 1,000 miles before getting rears replaced)
2) It was a straightforward job of replacing rears, also they don't do rear wheel tracking as they don't have the correct machine for four wheel tracking, when I returned they sent me to another organisation to get it checked, so again, I agree with your sentiment, but steering wheel doesn't lie.
3) £100 a tyre, so not exactly budget.
4) I'll revisit tyre pressure.... thanks.
5) Definitely not correct in this case, I've driven over 17,000 miles since I bought the car, so I'd know if it had been out for a while, it's one of the things I love about driving the SLK, how true and straight the drive is/was.
Cars, don't you just love em!!!!