Hi I've looked at a nice crv 2.2 diesel on the mot history it says rusty front sub frame but not structurally affected is this something to worry about if so how big off a job to change it

Hi....welcome to the Club

I wouldn't worry too much about that as long as it hasn't come from somewhere such as the coast or flood plain areas in which case the corrosion could be excessive.
Otherwise it has been noted as an advice more than likely but even so it is worth looking at how bad it is, if its surface corrosion then fine and it can be treated, anything deep and metal peeling away then maybe worth walking away from