Slice the squash in half length ways and set, flat side down in a baking pan.
Put the water into the pan.
Roast for approx one hour (test that it is soft using sharp knife).
Remove from oven and allow to cool for 10 minutes.
Remove the seeds and discard.
Remove all the soft flesh into a mixing bowl. Mash (it should mash easily) using the butter and the salt and pepper.