[NANNYSOO]

Hi  I'm new to site. From South Wales. Hope I'm  doing  this right.  I have this growing in my greenhouse. I planted a Melon seed, hoping to grow one, but a fellow ALLOTMENTEE  is quite adamant it's a pumpkin. Any idea please. Thank you.

[Kleftiwallah]

Good evening NANNYSOO, 
give it a few weeks, then ask again.  It may become apparent just what it is.

Cheers,   Tony.

[Nobbie]

Looks a lot like a round courgette to me. The stalk looks a bit too chunky for a melon. How big is it at the moment?

[mumofstig]

The stalk looks a bit too chunky for a melon.

That's the first thing I noticed  unlike a melon stalk - unless it's a Shark Fin Melon 

[Nobbie]

This looks very similar
https://www.magicgardenseeds.com/The-Good-To-Know/Round-Courgette-Rondini-%27Tondo-di-Nizza%27-(Cucurbita-pepo)-A.CUC07-

[New shoot]

unless it's a Shark Fin Melon 

They start off pale green then go deeper green with random splashes of white, so it doesn't look right for that either.  I think round courgette or some sort of pumpkin as well.

[Subversive_plot]

Yes, pumpkin or squash (courgette).

[jambop]

Is it a secret plant ? Because you are not showing us much of it
One way of telling easily if it is a melon or squash of some sort is whether or not the plant has sent out runners from the original point of growth. Courgettes generally grow directly from the main stem melons, pumpkins and squash that I have grown always grow their fruit on runners that emanate from the main plant and they normally they produce several of these. Again generally you will find a squash will ramble if you let it courgettes, in my experience, although they can produce a large plant they do not send out long fruiting shoots.