Hi @annie_andrew,
A warm welcome to the Bunnings Workshop community and thank you for your question.
If you can upload a photo from further back showing the gate when it is latched, this will certainly help. I'll need to see the gaps between the gate, the posts and the ground to get the whole picture.
When you say the ground is clay and the swelling allows for movement in the posts, is there concrete at the base of the posts? Is it possible to get a photo of the base of the posts?
If we can reduce the movement of the posts, any repair will be much more successful, so we should explore that option if possible. Once I've seen the base of the posts, I can offer recommendations on this.
When you say it is difficult to open, does the gate scrape across the ground at all? If this is the case, then we should also look at the hinges on the gate to see if they can be adjusted.
As you've mentioned, the D latch is quite rusted. If this affects the movement of the latch, then cleaning it up as much as possible, or replacing it altogether would certainly be worthwhile. You can give it a clean with CLR and a wire brush to reduce the amount of rust. Alternatively, this Pinnacle Stainless Steel D Pattern Dual Opening Gate Latch would be a good replacement.
Ultimately, the issue with the screws can be fixed either by shifting the tongue and d latch up or down or by attaching a piece of 32 x 3mm Aluminium Flat Bar over the holes with metal screws and then screwing the latch back in place.
Once you've provided some further details, I will be happy to assist further.
Jacob
