In reply to MonkeyPuzzle:
Elan Valley is perfect backpacking country, I've done several routes there and am always happy going back.
The 'new' bothy at Lluest Cwm Bach on the N.E side of Craig Goch res is absolutely lovely, someone regularly goes in with a wheelbarrow of wood, I suspect because they don't want bothy-goers raiding the little copse just by the bothy. It's quite spacious, it was very clean when we were there in June, and there's a toilet which you can flush using the rainwater collected from the roofs. We were planning on one night there and another at the sheepfolds at SN870689, but enjoyed the bothy so much we did a circular walk the second day and went back to the bothy for the night!
There's also a nice spot at the deserted farm at Ty'n y Pant, SN926632, though no water. You do get to walk past the little dam that Barnes Wallace blew up when he first started thinking about dambusting, on the way in from the Visitor Centre.
If you're going by car, they're very relaxed in the visitor centre and I've left my car there several times, just left the details with the guys at the desk there.
It does rain a lot there (there's a reason the land was bought and flooded to provide Birmingham's water supply), but it's wonderful country.
Photos of the bothy here, including pictures of the stray foxhound puppy we rescued and carried out the next morning in a makeshift papoose!
https://get.google.com/albumarchive/107059971728414503961/album/AF1QipMO-u9...