Hey, thank you for the reply but it wasn't actually within the styles in the end (I'd spent AGES looking around there prior to posting here).

I actually got to the bottom of this last night an it turns out the solution was in the bullets and numbering options.

In case anyone needs a solution in the future:

Go to Format ->
bullets and numbering ->
Ensure the relevant style is selected ->
customize ->
Ensure only heading 1 is selected (at least this was the case for me, if you wanted to remove the space in other headings select those) ->
find the "Follow Number With" option and choose "Nothing".

That'll sort it out!