Greetings to those not drifting around at Hamble! I am in the process of re-stringing control lines and wondered if anyone had tips on how to balance the shroud/lower cascades.  My worry is how to keep everything exactly the same length port/starboard to avoid the mast being out of plumb (i.e. leaning one way or the other?)  Is it just a case of trial/error/adjustment or is there a technique? 

Trial and error.
 
With the best will in the world the chances of the fittings on the boat being 100% symmetrical are slim so even if you're ultra careful with the lengths it could still be out.

 Thanks Chris... I'll set aside a few hours then!!

One technique I used when I was restringing the main shroud cascade was to cleat off the main halyard at the point where it just touched the deck by the shroud on one side and then adjust the other side's cascade till it would just touch the deck there too, at which point your shrouds are the same length and you tie the final knots.  Saves a lot of fiddling around with tape measures & spirit levels.
 
Andrew 3511 

Andrew, I like the sound of that technique and will give it a go making sure the boat is sitting square/level on the trailer and mast is roughly upright first I assume?!

Technically it should still work even if the boat isn't square & level but yes, initially get the boat level and the mast set up roughly where you want it - thinking about it logically you want to be setting up with the mast as far forward as you will want to go as that is the maximum length of your shrouds so is the tie-off point.

 I would have the mast slightly back from fully upright, as you will not be adjusting the shrouds whilst under full tension.  This way when the rig is pulled upright you will have more tension and therefore travel on the system, for when you are fully raked back.
 
 The other option would be to adjust the rig with slightly over maximum rake and the shrouds fully tensioned on the control line, this way when you let it off the rig should have enough movement to tension the rig for all conditions. (this also depends on the range of travel that you have available.)