Well done to you James, for taking in a cat others may well have passed by.
You're doing the right things, and Liz has excellent advice to give.
We buy our Zylkene online, but think you can also get it at Pets At Home as well as from your vet. It's brilliant stuff for calming skittish cats.
We adopted two semi feral girls in November last year, at 8 weeks old. They were wild, and they'd hide all the time. (Still are on occasion, especially if we need to get them to the vet).
What we found helpful (and I don't know how feasible this will be for your circumstances) was to clear out the room we put them initially. You need to limit the amount of hiding places, and essentially give her the confidence to own the space. The best "hiding place" is a roomy cat carrier, cage or cardboard box with her bed in. You could put a blanket over the top to make it more cave-like.
We didn't put food down and leave it with them. We made an event of food, so that they began to associate us as "food givers." When we were going to enter the room, we used to knock at the door and call out - I used to say "Hello Gurly-Wurlies" and OH preferred "Foodies - foodies" but whatever you feel less daft saying, and which you can happily call out for her.
It will give her advance warning you're coming in, but also, that you're sourcing her food for her.
We then put the food down a little way from their hidey hole, and we'd sit down and read or talk or watch catch up TV on the tablet, and wait for them to feel comfortable enough to venture out. Hunger is a great motivator. After they'd fed, we'd remove the food and just leave water. Overnight, we'd leave out a scant handful of dried food to snack on overnight - not much though.
When it wasn't "mealtime" we would go and sit in their room, and would put a few treats near to the entrance of the "cave". After a while they'd dart out, steal the food and run back to the cave. It took about 4 days before they would warily venture out and came anywhere near us to get the treats. However, we were dealing with much younger kittens than yours, so don't try to place a time limit on your achievements, as they need to go at her pace.
When they were eating, we'd very gently extend a finger towards them, not touching them at first, just letting them get used to the idea. It wasn't too long before we could actually touch them, but they'd run away immediately.
We would spray our hands and clothes with Pet Remedy spray, and also spray their bedding and the carpet. Unlike Feliway (A similar product which they recommend you don't allow to touch a cat's fur or skin) it isn't harmful if cats get it on their fur, and they can - and often do - lick your hands when you've sprayed them (but let it dry!) The smell is very attractive to cats, and calming. You'll pay about £20 for a 200 ml spray at PAH, but the last one I bought online was £9.99 for the same size product.
Pet Remedy plug ins are also good. Again you can get these from most vets, or from Amazon and other online outlets (usually a good deal cheaper than at Pets at Home and the vet) We had one in the girls room, and one downstairs for our other cat, who was a bit puzzled and worried by what was going on in our back bedroom.
Our girls are now 6 months old, and have come on in leaps and bounds, although they're still not cats we can pick up for more than a nano-second.
Having said that, they're affectionate - but very much on their terms. They will cuddle with me and OH and our other cat, and they really enjoy a little snuggle session - they just still feel very intimidated by feet and by being picked up.
We found this advice really helpful:
http://www.theluckyfew.org/site/feral_cats.htmlThe task you've taken on may not be an easy one, but it will be so rewarding when you win her trust. And you will.
There are lots of opinions out there that once a cat gets beyond the magic 8 weeks and hasn't been handled, then it is feral for life, but thankfully, in practice, many have succeeded in proving that theory wrong, and Liz is a perfect illustration of that, with her incredible crew of strays and ferals.
We had our doubts, but we kept the faith and it paid dividends. Look forward to hearing how you progress.
What have you called your little lady?
What colouring is she?