Hi and welcome to the area!
I have a child at Ivy Lane and have been happy with the standard of education given and the atmosphere is very positive and caring.
But more importantly, my child is happy there.
They have on-site facilities for full day care - a nursery (Happy Days), Breakfast club and After school club.
Geographically, the 3 schools you have seen are quite far apart. I would suggest you take school runs etc into consideration. Traffic at school run time can be slow, so if you have the option of walking, I would factor that in to your decision! (It doesn't seem important, but when you are doing it twice a day - often in the rain - every. single. day. it really is important!).
I know children who have been to St Marys, it still has a good reputation although I believe there was a bumpy period when they had a change of HT (this could just be gossip though).
Also, if you are thinking of staying in Chippenham long-term, the secondary schools are excellent.
Coming from a large primary (like Ivy Lane) is really helpful as they are used to being in a large school. Also, they are more likely yo be placed with friends in new classes. (I appreciate this is a long time in your future, but these things have been really helpful for my DC).
If you have any Ivy Lane specific questions, please do ask. It was so difficult when we moved here, looking at schools and nurseries, I'd be happy to help if I can.