Hello everyone - thanks so much for your replies.
So I repotted and sure enough there were many dead/rotten roots:-( And a few OK looking ones. So I followed the instructions above and cut them off and repotted in a container with new holes I made in it. The new medium is probably not ideal but it was all I could get hold of here - it's proper orchid medium and is bark-y, but has some soil stuff mixed into it too. i haven't packed it in very tightly.
When i unpotted it, sure enough there was a weird spongy thing in the middle of the pot, so pretty sure the orchid wasn't bought from anywhere other than somewhere that just smashes as many out when they are in bloom, not really caring if they are going to die soon after. As Ihave only had it 3 weeks, the rotten roots were clearly there when it was bought
I might have buried some aerial roots when i repotted it, but anyway I have done my best and we'll see what happens.
PS i did leave the flower stalks on in the end.
Fingers crossed it recovers - thanks again for your help. See pics below for the surgery and end result.