Thanks for the help
Turns out the magnets were the wrong way around
I put them the other way and bingo, I kept the sensor the same distance and every thing seems fine.
Word of warning tho'
Dont bake the epoxy resin to harden it if you use glue for the magnets,
The heat weakens the strength of the magnets
I didn't realise this untill I was informed by someone in the know.
Evey day's a lesson.
For my set up, it didn't matter the sensor still works fine