I used someone's post in the end to solve it. Below is what I did in the end.
In the form the parameter you wish to lock as "read only" can be triggered by a custom made "true/false" parameter.
1. Right click on form and edit
2. Find and select the parameter you wish to make read only
3. Under behaviour there is an option to use a custom parameter to control this parameter. If you try to select a parameter you may notice you will only have "none" to choose from.
4. Create a custom "true/false" parameter, not numerical or text.
5. Repeat step 3 and select the custom parameter you just created
6. Create a simple Ilogic and write the custom parameter to " = " (equal) true or false.