John if you are here in the UK you are way ahead of the rest of us. I might be thinking of starting my courgettes (which if let grow will turn into marrows) in 6 weeks time.
Wait until the last frosts have gone then start a few off in 3" pots on the windowsill.
I'd plant them on their side, about an inch deep in a 3 inch pot, in late April. I'd keep them indoors on a sunny warm windowsill. If its still chilly on the said windowsill, I'd cover them with some bubblewrap for the first week to help them germinate. Once they are a couple of inches tall and the weather is warm, they can be transplanted into the ground. Probably late May early June.
Alternatively plant them ouside directly into the ground in late May.