To be honest I've never tried sowing squash direct, but I've heard of folk who use hotbeds managing to grow them in cooler conditions. I find they need decent temperatures to germinate and careful hardening off. Mine are currently in pots outside my greenhouse at home, I'm waiting for the night temperatures to be consistently over 10degrees and for the nematodes I applied last week to get going before risking planting out at the allotment.