First of all can he pay the 75 euro a week. Is this why he is refusing? There's alot of difference between 5 years ago and today no matter what he signed, if he honestly cant afford the 75 a week then there's nothing you can do.
If on the other hand he's just being an idiot and he can pay then you can take him to court and it wont cost you a penny. You dont need anyone to represent you. I'd take the papers that were signed anyway but you dont really need to take them with you. You could find by just making the application he'll decided to pay he might not want the hassle of going to court, and if he doesn't turn up happy days for you. If you know he will turn up then it could be a good idea to write down your incomings and outgoing for the judge to see just incase he asks. Depending on what he earns i would say you could get more than the 75. My ex earns 45k p.a, has a very low mortgage, he built his own place and the land cost him nothing, i was receiving 70 euro a week and when my son started school he was meant to pay 1/2 towards everything and he refused. When i went to court the only thing the judge asked me was how much he was earning...i told him 45k per year, he said i'll award you the full amount of 150 euro a week.
Now some people said i was stupid and others said i was being fair, but i actually told him i didn't want 150 euro a week. Must say the judge did look at me a bit weird lol All i wanted was the payments increased to 100 euro a week, and thats what i got. I wasn't out to earn from him i just wanted him to pay his share towards our child education costs.
If you go to your local court office and make an application and say you want to make an application for maintenance they will fill out the forms there and then and give you a date, usually in 5-6 weeks ( this might have all changed it was 4 years ago i was last in court ) They will then send a letter to your ex telling him the date and time that he has to attend. And thats about it. ( it wont cost you anything )
What ever happens they wont make him pay less than the 50 euro a week he is already paying, so if there is no increase in your payment you might not have won but also you would not lose anything either, so it's worth doing.
Good luck.