Can someone tell me what this bolt is for

The one on the right hand side that bolts into the end of the steering arm casting on the hub?

It sits halfway out as though its adjustment for something, and has a nut on the other side that also retains the disc mud guard.

Cant see what it adjusts, or even does apart from hold the mud shield on and its a massive bolt just for that.

Same on both sides too

I think you are talking about the lock stops. The idea is you adjust the bolt in and out so when it touches the swivel housing Bolt on full lock nothing else touches, on mine its the tyres that are the limiting factor. Obviously the further out they are adjusted the poorer full lock will be.

Tyre needs to miss radius arm by 20mm on full lock

Ah ok so I put the car on full lock (until the tyre is 20mm from the arm) then adjust that bolt until it hits the swivel housing?

Not an easy one to answer. Depends entirely on your tire size. for example, the workshop manual states: 'turn the steering to full left lock, the clearance between the left tire wall and radius arm should be 54mm when fitted with 750 x 16 tires and 51mm on all other models'. 'The same applies for the right hand side'. If you have put bigger springs on or massive tires, it will affect the goemetry. Just go with 51mm and make sure it doesn't scuff anything. Not MOT tester will notice unless it's a main dealer.

SJW2511 said:

Ah ok so I put the car on full lock (until the tyre is 20mm from the arm) then adjust that bolt until it hits the swivel housing?

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Thats basically it, what you have to remember is when the vehicle is moving at slow speed ie shunting on full lock that gap will close to almost nothing, if it touches one side just wind the bolt out 1/2 a turn the opposite side until you are happy, mine just touch a tadge on full lock i reverse, nowt to worry about.

Just reading the reply ref 51mm thats way to much, I set hgvs up with less than that!

You will also get more lock with vehicle jacked up than sat on the floor, I just do mine on the floor takes less time.