I recently found an advertisement for my job at a higher wage than I actually get, before I confront my employee I want to get some information. I have been with the company 12 years and have never been on maternity before, I plan on taking 9-12 months off all together but in the advert there is no mention of this being maternity cover. I know that when I come back they have to offer me a job at the same wage that I am receiving at the moment but I am so angry that they can offer a higher wage then I already get?
Not illegal, no. It's perfectly allowed for employers to advertise vacancies at higher rates than existing employees get. Paying people different salaries for doing the same job could leave them vulnerable to an equal pay claim if the people on a higher rate are male, for example, but in itself advertising higher isn't unlawful, although for obvious reasons it's not very sensible given the likely reaction of existing staff members.
I think you need to speak to your manager and get clarification. It's not unusual for staff employed on a fixed term contract to get paid slightly more than permanent staff, but if as you say this is being advertised as permanent then I think you are reasonable to raise it with your manager and say you are concerned to see what appears to be your role being advertised but not as maternity cover for you and at a higher rate than your salary. Get their explanation and take it from there.