Steve, you could increase the outer margins or perhaps simply redesign the layout of your page (which has advantages when the page must display on a mobile)

Thanks Esahc

Very much appreciated. In the end I changed the margins and it's worked out fine. Just a few related questions

1 - When I changed the margin, the maximum figure I could set was '250'. Is this by design or is the 250 related to something other setting?

2 - I couldn't see the padlocks on the page layout as per your screenshot above. Is that because my page is two rows by just ONE column (see screenshot)?

I think 250 is just the arbitrary maximum, the trouble with margins  is that they are also imposed on mobiles (& 250 is approaching the width of a mobile).

Yes the drag bars appear when you have more than one column.