Hi Nicko,
Wow how silly of me to miss that. I need to up my medication.
What's actually happened is that someone has changed the chain and it wasn't long enough so they undid the nut on the engine side of the side plate and removed the assembly and then as it's got a square hole they've rotated the adjuster 90 degrees left so that it points down instead of forward.
I strongly suggest you get the correct chain and that way you will be able to use the tensioner again and have the correct adjustment on the cutter chain as that sprocket is moved further away from the top PTO sprocket as you grind the reel and bed knife as the reel then has to be adjusted downwards due to lack of metal on the reel blades thus the tensioner has to be slightly rotated down wards to accommodate the extra chain length required.
Cheers,
BB.