strange tyre wear issue

When I had a puncture mended on Friday, I took the spare off the NSR and before putting it back in the boot, I noticed a visible line about 3mm wide around the circumference of the tyre. You can also feel the line as slightly raised above the tread pattern. It's slightly off from the centre of the tread pattern.

This tyre has been on the OSR for about 21 months (at least since I owned the car) and is worn almost down to the limit. It got relegated to the spare about a month ago as the existing spare was brand new. When I had a puncture on the NSR on Wednesday, the old tyre then went back onto the NSR. Everyone with me so far? :-)

I'd take a picture of it and post it up, but the spare is in the boot of my car, which is currently languishing in a garage in Scarborough awaiting all the petrol to be drained from its tank and fuel lines etc.

The old tyre is visibly worn more on the outside than it is on the inside.

All of this suggests a rear alignment problem. Now - about six months ago, I asked my tame mechanic to sort out the steering. To go straight I used to have to aim at about 12.30 or 1 o'clock on the steering wheel. He said that the front wheels were 0 to each other, but off by about 6-8 (iirc) to the rear wheels. Does this make sense? I have only a limited understanding of tracking, especially 4wheel tracking.

I guess it's possible that the uneven wear was caused by the tracking being well off, and this funny little line is part of that?

I'll keep an eye on the two tyres on the rear now to see if any sort of pattern emerges - IF I get my car back! Fingers crossed.