How to get moist goo in layers of chelsea buns
Traditional recipes don't seem to aim for goo, and most of the ones you can buy aren't gooey (UK, limited sample of sources). You can try increasing the quantity of spiced, sugared butter (or even mixing a little milk and/or egg in with it) but you may be better off adapting that aspect of another recipe that is supposed to be gooey. You ought to be able to find a cinnamon roll recipe (for example) that states it has such a filling, but swap the cinnamon in the filling for mixed spice. I've been thinking more about this, and about pains au raisin, which tend to have your goo in them. According to Wikipedia they use crème pâtissière. 

From my limited experience making Chelsea buns, adjusting the cooking time/temperature won't help - you still want even the middle ones to be cooked through.