Thanks for the good suggestions everyone
Since my frustrated post we have found a few strategies to get him up the stairs each night, usually one of the five or six things we try will work (a 'race', a 'counting the steps', a 'left foot or right foot' game, negotiation - an extra book if you walk all the way, and a few other things involving chasing me/DH for a soft toy which gets thrown progressively further up the stairs).
The private physio sort of sat on the fence, said they do try and make it fun in therapy but day to day tasks can't always be fun, but if he's obviously too tired not to force him. This still leaves room for interpretation though. Anyway feel more positive about things after the physio session, lots of what they said made lots of sense. Thanks everyone for your suggestions!
and I love that story, Everyday, thank you for posting it.
Next thing (sorry!). Has anyone dealt with quite contrasting opinions between experts? Our NHS team are very pro-orthotics, pro botox and casting etc. But our private physios are very anti all of these things, preferring instead to work with the muscles as they are - saying that splints don't allow muscles in the leg to be used (and so strengthened and lengthened), that the weakness from serial casting isn't worth the flattening of the foot you get, and in general have an approach which is much more about building up the strength of the core and targeting the muscles in the legs which control his walking. They did concede that the casting was good to get a flatter foot, it has given them a window of opportunity to strengthen that muscle, although they lament the loss of strength which they say is very, very common, and hugely underplayed by proponents of serial casting, and have explained that now his centre of gravity has shifted with the angle of his legs it's now making him even more unstable and weak than before as he's essentially having to learn to stand/balance/walk again. 
I'm inclined to think what they're saying makes more sense, the orthotics have always struck me as counter-productive in lots of ways. Yes, he may be able to walk better with them, but it's definitely come at a price (lost tone, increased tiptoe walking, lack of use of various muscles groups) - and ultimately I'd like him to be able to get up every morning and be able to walk independently, on his own, without aids, rather than get up and strap some plastic onto his legs. It's difficult to know which path to go down as neither really complements the other. It feels like the NHS just want to be able to tick a box that says 'can walk independently with or without orthotics' to make him technically independent, without really working to make his quality of life the very best it can be.
Has anyone found similar? And apologies for the second huge post in the space of a week!