when i grew marrows a few years back i left the leaves and stems alone, the marrows will still grow well.
Are they (stems and leaves )getting to leggy then?

Yes they are very "leggy" (thanks for the new term).
They are taking over other plant and restricting there
growth, sunlight, and water.

Was wondering if could just trim it all up?
Will it hinder progress or marrows that are already
developing?

Thanks or your help!

Alex

Personally, I wouldn't trim the leaves down at all as this is how the plant gets a lot of it's energy (from the sun) However I would cut back any distressed looking leaves as these won't be contributing at all.

Marrows are an extremely large plant anyway for the relative size of their fruits.