Performing IDW interpolation in QGIS?

I am fairly new to to QGIS and I am trying to create a contoured map from point data. The points are regularly spaced at 3.5 metres apart and whenever I try and run the IDW interpolation in QGIS 3 it creates a layer that with the minimum and maximum value the same and nothing is visible in the screen.

I have tried the same in version 2.18 and it does create a layer with different values, but it is gridded and not contoured.

I have managed to use these tools before with other data sets, but I am properly stumped. I have tried different settings and values but nothing seems to give any different results in either version.

Picture 1 shows the layer from QGIS 3, picture 2 shows the settings from 2.18 and pictures 3 and 4 show the results from 2.18 being gridded with the red points laid over the top

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