Our french insurer (Crédit Agricole) didn't ask us to fit anything. We do fit a hitch lock that came with the Alko jobby and I like the look of the leg locks. I too would be devastated if ours was stolen.

I've found some here that fit 19MM bolts. Will that fit?

The corner steady nuts are 19mm. The SAS units are held in place on the nit by a single tapered grub screw that is set with an Allen key.
I would suggest that putting the tip of a drill into the flat of the bolt where the grub screw is going to sit would be a good idea - probably a 5mm drill going in about 3 mm would be sufficient. This would help prevent the whole thing being drifted off with a few well placed blows from lump hammer.
My steady bolts have a hole right through and I'd sooner the SAS item used that with a through and through fitting which would be far more stable.

The Milenco items are supposed to prevent the use of mole grips or similar being used to turn the steady system by just gripping the thread.

Just to update this - picked up a pair of Milenco corner steady locks - quick and easy to fit, block rotation of the winder shaft and minimal exposed metal - just what the doctor ordered. Come with 3 numbered keys (useful) in large blister pack that is not - hate these knuckle slashing packaging things!!!

We got our 540GT last summer and sadly we have become very security conscious at home. We have had the drive modified so that our van is now behind a garden wall and security posts. This is along with a clamp on each wheel and the hitch lock. This is because we saw the at first hand the terribly traumatic effects on family friends of some low lives stealing their van (not Eriba) the night before they were due to leave on holiday next morning. It was loaded up ready to leave but was clamped and in a locked compound. They came in across fields at the rear of the compound, ripped down security fencing and smiled and waved at the CCTV. It was made worse for us as we also knew the compound's owner. Everyone was devasted, the Police showed very little interest and the insurance company took a lifetime to make any payment. The family are still really upset nearly twelve months later and say that they will never risk owning a caravan again.
I think this has made me a little neurotic and determined to do whatever we can to protect our lovely van from the scumbags. We have not bought steady locks as we read somewhere that thieves simply bent the steadies up and drove off. However this thread has made me think again.
Mike T

It all seems to reinforce the view that if they're determined enough, not a lot will stop them.