Is there a substitute?
You could use acorn or butternut!
Most winter squash have a pretty thick skin, with a few exceptions: delicata is one. Another one is Carnival. Both acorn and butternut will work great with this recipe, but you might find the skin too thick to eat. Here's some additional information on winter squash alternatives: http://missvickie.com/howto/veggies/wintersquashtypes.htm
I'd use acorn and peel it. Butternut seems a little sweeter than acorn and delicata isn't particularly sweet.
I ended up with the acorn. Should I cut a bit thicker slices if I'm not peeling it? Many thanks from a novice cook :)
I like to work with the shape of food as it's presented naturally, so I would consider cutting it in half, scooping out the seeds and slice it in slices like a cantaloupe and going from there with the recipe. You could consider adding some dried fruit like raisins and dried apricots all cut up too to the relish too. I'd love to hear how it goes for you.