Thanks all for the info.
@Timb - what rear hub are you using?
I'm currently running RD2 cranks with a 3/32" chainring bolted to the inside. 1/8 chain. 1/8 sprocket. I think Spa fitted a 107mm BB. Hub is Sturmey Archer HBT30.
The chainring has visible signs of wear on the inboard-side teeth (and chain was noticeably noisy on Friday's Alternative E&SB 200) which suggests chainline could be improved by moving the ring out by another mm or two. I wonder if this could partly be the reason why I can't get acceptably even chain tension, although I think the crank spider also isn't concentric.
I think I should take Rogerzilla's advice on another thread to use a 2ft ruler to more accurately check my checkline!