Progression Opportunities while on Maternity

I have worked for the NHS for 7 years in the same position. I have applied for 2 (progression) promotion opportunities in my department before going on maternity and I wasn't successful. My desire to progress is well know to management. While on maternity, another opportunity came up and everyone in the department was informed via work email. They didn't inform me about it and went on to appoint someone with half my experience within the organisation. I have engaged HR who have been evasive (not putting anything in writing). They say that it is "best practice" to inform staff who are on maternity of progression opportunities. In their opinion, I have no grounds for discrimination. I'd really appreciate some solid legal perspective. I think my conclusions are based on emotion and ignorance at the moment. Thanks. Ronnie