Aye mick0905,
Slacken off the chain adjusters to their full extent, with the rear axle hard against the inner end of the swinging arm forks. Then tighten both nuts alternately until there is no slack in the adjuster threads, with the axle still resting on the back of the forks. Now turn each adjuster nut by 10 flats and continue alternating between the nuts with equal number of turns, until you achieve the correct tension on the chain. The pitch of the thread is constant, so as long as you start from a common position and turn the nuts by the same number of turns, you should draw the axle backwards evenly.
To check, take 2 un-warped 4" x 2" wooden battens/peices of alloy or box section steel, long enough to extend from beyond the rear wheel to beyond the contact point of the front tyre with the ground. Drill 2 holes through so that 2 coach bolts with wing nuts [or similar], will hold the battens against the rear tyre. Providing the battens are true, gripping the rear tyre either side of the rear axle and you have tightened the chain adjusters equally, the distance between the front tyre sidewall and the battens at the front should be equidistant.
Sorry for the ramble, but hope it helps.
My regards, Bill.