I recently bought an 8m Peter Lynn Synergy used (almost new) from ebay.
Today I found out the left 2 lines were a few cm shorter than the right 2.

The way the lines were connected to the bar when I got it was:

• red line with red sleeving and red line with grey sleeving were attached to the center strap leaders
  
• blue line with red sleeving and blue line with grey sleeving were attached to the leaders on the bar ends.
  

It turns out that the lines with the red sleeving were the same length as each other, but a few cm longer than the other 2, which were also the same length as each other.

I have "re-strung" the lines so that both lines with red sleeving were attached to the center leaders and both lines with blue sleeving attached to the bar ends.

Is this the correct way to attach the lines for this kite, or was it right in the first place?

Also, does the fact that the red-sleeved lines are longer suggest that they were stretched out before I got the kite?

One side might be longer do to the stretching process when he had the kite first and probably over time he's one side dominate. With 8m you flying in high winds to start. If you're not picking about colour than I would use your shorter two lines for the power and the longer ones for the bottom. Arcs use the same weight for all for lines.

Most important is to match line length. Measure them relative to each other.

If you have 2 long and 2 short but they match up. then put the short pair on the front and long on back.

Nothing to add here

Thanks, all!