I totally agree with DD. Start them off in your propagator in March, then remove when showing an inch or so of growth onto a windowsill. Try using a flap of cardboard from a carton, bend one of the longest ends to a right-angle, cover the rest with tinfoil, and place the bent flap under the seed tray at the back so the foil can reflect the light from the window and stops them getting leggy. This starts the hardening-off process too. This should take you up to mid-April or so. Keep an eye on the weather and put in your greenhouse towards the end of April, but be aware that a cold snap will harm them, so bring home again if that happens.