I've never grown these before so would someone be able to answer me a stupid question. The little buds on this picture - are they the flower buds that turn into the pumpkins?
Right - two answers here: Hundredweight are 'trailing' pumpkins and they grow very long indeed, so although they don't need all that much space between each plant (from side to side) they grow very long.
I usually plant mine 3ft between plants in a nice rich hole, with a bund around it, at one end of a bed and then 'point' them in the direction I want them to grow. If you want a huge pumpkin off the plant, then remove the growing tip when one fruit has set and is growing nicely. If you want several smaller ones, then wait for a few to set, then remove the ones you don't want and the growing tip. Keep them all well watered (fill the bund every evening, unless it rains like crazy).
As for your picture - its hard to tell what the buds are going to become, but you generally get more male flowers than female flowers, and these look like males at this time. (Female flowers have a little 'bead' at the base of the flower. When the flower opens and a bee does the business, the 'bead' is fertilised and will grow into the pumpkin.) Sometimes, after fertilisation, it starts to grow and then, for no reason that I can fathom, it falls off. That's why I say let a few set so you can be sure you will get (a) fruit.
You can pick the male flowers when they are very fresh and stuff them like courgette flowers and eat them if you are so inclined. Otherwise, just leave them, they will drop off by themselves after the bees have eaten the pollen, and, hopefully, fertilised a female flower for you.
There are probably other ways to do things, but this is what I do! Good luck.
Sounds like you are going to have one heck of a lot of pumpkins! (Or there may well be others on your plot who would like a plant. Why now have a competition to see who can grow the biggest? Measure the pumpkins (in about late September), around the equator and around the poles and add the two measurements together. The biggest number wins. I suggest two measurements because some pumpkins grow flat and round and others go tall and round ... if you know what I mean! - so this means other people can also have a go with whatever variety they have grown)
Oliver