What is the maximum load that the shackle should be able to withstand? The one I'm looking at currently comes in three sizes, with loads of between 1000kg to 3000kg.

I'd never thought of it in that way, but I assume that there can potentially be quite a lot of pressure, and I wouldn't want the shackle to break just at the wrong moment. Would the 1000kg one be big enough?

However, I don't recall either of my Ents kickers fouling the CB head? Fittings are pretty strictly controlled in the Ent so are the kicker attachments in the correct places?

Also it will be easy enough to reattach the kicker when the wind is light/moderate but in a blow it might be difficult on the water so a kicker that can be attached before launching might be a better choice if it can done.

The grip at the front of the centreboard is just in the wrong place for the kicker; it's fine once the board is down even a little bit. I had problems on the shore with the boat still on the trolley with the wind gusting and shifty; the boom moved across, kicker got stuck on the handle, and off broke the top part (the bit that was out of the case at the top).

Rupert: so you think the 2000kg should suffice?

I think that 1000kg will be more than enough. Sure there is a lot of purchase, but do you think you could feasibly lift 60kg, equivalent to a small adult with one hand using 4mm control line? Also think about the strength of other components in the system. Look at the final line strength, or final block type they will give you a good steer for strength of the system.

Enterprise 20361: When new, that had the lower kicker attachment as a U-bolt, bolted through the hull, with a backing plate underneath. It would take a lot of rotten wood and close to a complete boat failure for that to pull out! 

And if the boom is still the original, then the fitting on the boom could easily be in the wrong place. When new, it had a complex highfield lever system on the boom for the top of the kicker, to allow a coarse as well as a fine adjustment on the kicker lever. I suspect this was all replaced with a proper cascade system years ago!

Good point about the block types, they will indeed be the weakest link (most likely) but I reckon I could pull 60 kg onto my kicker line so, with a 16:1 cascade 960kg less friction losses (which will be a fair percentage) but it's the additional shock loads in a windy gybe that will be the issue and they could easily be double or triple the static load.

Not sure if the boom is original; it could well be. And yes, it has a cascade kicker which is attached to the floor.

My problem is that the kicker is already maxxed out before I get the right setting for stronger winds, so ideally I need to move the attachment point on the boom further back (obvs in line with class rules), but that makes it even worse with the centreboard interference.

I think the consensus here is that the 1000kg ought to be enough, but might be borderline; and the 2000kg would be a safe bet. Thank you for all the useful comments!