"I understand that with TUPE T&C's should be no less favourable than previous contract of employment but where does the boundary lie in relation to changes in job function/location."
This is commonly mis-perceived as being a guarantee of sorts that your terms and conditions can not be adversely altered. It is not true. The old employer had a right to re-organise, and this could have included materially worsening terms and conditions. Obviously, the old employer would had had to go through proper IR processes, but no employee is immune from change.
A better way of understanding TUPE is not to say that things can be no less favourable. Rather, it is the case that the mere fact of the employer changing can not of itself be in any way used against you.
Whatever rights you had, you still have. These rights do not generally include the guaranteed right to be permanently employed on your existing terms.