Do you have a pension statement for the year of your first mat leave to prove that they didn't make their employers contributions?
The fact that they have repaid the full contributions during your second maternity leave shows that they know they were wrong to stop them, but I suspect that you might need some evidence to show to them to make them make the contributions.
Has anyone had their pension contributions stopped by their employers while they are maternity leave? If so, how did you make them pay it back? I have 2DCs and after returning from my second maternity leave I noticed that they had stopped my pension (they said it was their policy to always do this when someone went on maternity leave). This rang massive alarm bells for me as it meant they hadn't paid it for the previous one either (which I should have noticed but hadn't). I queried it and they paid back the last leave but will not pay back the previous one. This means I've got a nine month gap in my pension. All the stuff I've found on the internet (direct gov and pensions advisory service) says that they shouldn't have stopped my pension just because I was on maternity leave. Anyone got any advice about getting them to pay it back?