I would be grateful if anyone could help with this:

If a commissioner remits a case to a tribunal having made a decision that a particular interpretation of the law is correct and the tribunal makes a decision unfavourable to the claimant is it possible to appeal to a commissioner again arguing that the previous commissioner's interpretation is wrong?

It's possible, since you would be arguing that the tribunal were wrong in law. The fact that they were following a Commissioner's direction doesn't mean that they were automatically right.

However, I doubt if you'd have much chance of getting leave. The chair considering the application would be bound to take the original Commissioner's decision into account, and unless you had very strong arguments that that case was wrongly decided, would almost certainly follow it.

You might have more chance of getting leave from a Commissioner. Commissioner's do not necessarily have to follow each other decisions, and there are plenty of examples of Commissioner's giving conflicting decisions on similar cases. However, the fact that the original Commissioner had been looking at your client's case would probably make it unlikely that you'd get leave. I'd think your application for leave would have to be very well argued to persuade a Commissioner that it was worth looking again at exactly the same issue that had already been considered.

Of course there's no reason why you couldn't appeal on different grounds. If you were able to get leave based on say inadequate reasons, I don't think you're then precluded from running an argument that the original Commissioner was wrong.