Hi Jody,
Super to hear from you, and I'm so glad that you are building the gorgeous Jemima an outdoor enclosure.
Oh dear, Ragwort is something to be worried about. Weeds are very clever organisms, and will surprise you with how tenacious they can be, but your plans certainly seem designed to stop it reappearing. The wire at the bottom of the enclosure will also prevent Jemima from digging down too deep and disappearing too.
The only thing I'm concerned about is the pond liner. I might have misunderstood your plans, but won't that stop water from draining down and out when it rains? And you can't tell me that it is always sunny in Derbyshire, because I've been in Wirksworth in the rain.
I once built a tortoise enclosure on top of a patio that was made out of concrete slabs. It was a rented accommodation and I couldn't remove the slabs so I built a wall of bricks around it and dumped about 8" of topsoil on it. A made a nice little hill in the middle for Doris to climb on and survey her kingdom. One day we had a torrential rain storm. I forgot that Doris was still outside until about 10 minutes after it had been raining, and I rushed outside to find the whole enclosure about 6" deep in water, with poor little Doris on the 2 or 3 square inches on top of her hill, just managing to stay above the rising waters. I had forgot that there was no way for that water to drain out, and i'm wondering if your pond liner could have the same effect.
The other thing to remember with the enclosure is how well tortoises can climb. If you are having wood walls then nail a little 1 x 2 or something on the top, to form a little lip that projects inwards and inch or so. That way if she finds something to climb up (and walls at right angles are always a useful way for them to make the great escape), she will come against that protruding lip and won't be able to go any further. I recently posted these photos on another thread, but thought i might post them here as well, because it's always amazing to see how well they climb. If you are having a wooden lid on it, then there's no need for a lip, but do make the lid on hinges so that it can be easy for you to get in and out. And make sure too that whatever you are using for the lid covering still lets in lots of natural sunlight so she gets lots of UVB.
Untreated wood in the ground will probably rot in three or four years, but you can replace it or treat it with something to preserve it. I'm lazy and use tall log roll, and that certainly does rot after a few years, but it's so cheap and easy to replace that I don't bother doing it better. Your enclosure will be much posher than mine!
The only other thing I can think of is to make the enclosure as big as your space will allow, as tortoises do like lots of room and things to keep them interested, and knowing you, I'm sure it will be a great enclosure!
Nina
[attachment=1]Tortoise climbing fence.jpg[/attachment][attachment=0]Tortoise climbing wall.jpg[/attachment]