P.S.

I should have mentioned.

I purchased a "Load leveller", CRA-LL450 from Chester,

two 8mm bow shackles:
2 x Tested 8mm Bow Shackles SWL 0.75 ton (Lifting Gear) | eBay

the bolt is the right size to slip through the links on the chain provided by Chester.

two 12mm bow shackles:
2 x Tested 12mm Bow Shackles SWL 2.00 ton (Lifting Gear) | eBay

The bolt will pass through the eye of the above shackles and has a large enough eye to accomodate the two ends of a 1m sling:

2 x Duplex 1 meter x 1 ton Webbing Lifting Sling Straps | eBay