Hi Slaps
I have seen some companies which only had approval from Head Office for a temporary role, advertising for a permanent position, to attract candidates who would not be interested in the temporary job.
The key question is why is the job temporary. If it's definitely going to end after someone comes back from maternity leave, then they were wrong to advertise it as permanent, but I doubt if they have breached any legislation.
If as Leo suggests, that they are not confident in you, then stay in your current job which I assume is permanent.
Brendan