I ended up going with the B+Q orchid mix because I was desperate to get it out of that hideous state it was in. Rosie, I think you're right about it being less airy. I'm going to order from that site and get some nice airy bark chippings that my other orchid is in. When that one is due to be re-potted I will do them both together into larger pots if necessary.
The B+Q compost looks like soil but there is some spongy stuff mixed in and a few bark chippings.
While I was there, I got some larger clear pots (6"). I was going to put this orchid into a 6" pot, but ended up putting it into a small 4.5" pot. It looks on the large side for this pot! The roots are tucked in quite a lot, but I did read that they like to be on the snug side. When/If it climbs out of this one, I will upgrade to the 6".
As for the roots: there were a lot of them. Some were dry, papery and brown and just fell off. Some were mushy and slid off in my hands and exposed the wiry inner root. I snipped these back further along the root. There were lots of brown roots which felt firm. I decided to leave them for now, as I wasn't sure. I saw this thread on Orchid boards which suggested these are healthy roots covered in velamin.
Here are some pictures of the Orchid now. I'm still uncertain about one of its leaves which is literally hanging on for dear life
I am encouraged by the small leaf at the top which indicates the plant is still growing. What this orchid has been through is tantamount to abuse but it might just make it! Thanks so much for your advice!
Repotted:
Damaged leaf:
