I would suggest that your terms are those that are suggested by PhysioFirst. That is that all clients must pay for their treatment at the time of consultation. If you enter into any arrangement whereby you are running a billing service for them then you are going to end up where many found themselves in this recent BUPA tender. i.e. slowly but surely in a position of having to tender.

I think everyone needs to use the statement from PhysioFirst as the direction and reason why they take that stance.

Of course you could offer to hold invoicing them on behalf of their clients and run their billing for an additional fee, say £20 on top of your standard treatment fee for each session?! See what they say to that