After having a look at it, I concluded the bonnet was touching it and forcing it slightly too much towards the fan. So I remounted it further down, but closer to the fan.
This was a bad idea!
Turns out the bonnet wasn't catching it, but the mount bracket I made
was flexing at speed and pushing the rad back.
So I had to fix the rad again (even bigger repair this time) and made new brackets for the rad and mounted the intercooler while I was at it. Removed the mechanical fan and mounted twin electric ones in there.
So there we go, no problems with the rads, just me being a muppet!