Edges curling

[HarryThePrinter]

E3 V2
EN-PLA 205C/60C (first layer 210C using Prusa slicer)

After about 30 mins - 1 hour some edges curl on a large rectangular part.

I think expansion/contraction is the problem, but I don't know how to fix it.

There maybe poor adhesion of the first layer, but I've waited 25 days for the BL-Touch I ordered from Creality UK so I'm stuck with manual levelling atm.

Any suggestions?

thanks
Paul