ID can't create circular guides like Illustrator, at least not yet, so the best you can do is to make your own circle with appropriate stroke to use as a guide and put it on its own locked layer so you don't put content into it. You may also need to add a die line, which should be a spot color and set to overprint, to indicate where the trim belongs.
Setting up as a square with an additional bleed is probably a good idea -- at least it will produce a Trim box in the PDF at the correct position which will help the printer.
Thank you for your reply.
Regarding the dieline the printer has stated they do not require this.
Many printers require slightly different specs, so I am just trying to find the best and most convenient way to set up my document ready for the printer. As my design background is one colour, my first idea was to create a background layer of square - 110 mm x 110 mm and then place a circular dieline 100 mm diameter on separate layer with spot colour and set to overprint, this showing clearly where to trim.
You mention about making my own circle with appropriate stroke to use as a guide. does that mean in my case I should set one circle - 100 mm for the design and a separate circle shape of 100 mm diameter with a stroke of 5 mm with distinct colour - this representing the bleed and place it on separate locked layer and name it bleed line.
If the trim is 100 mm diameter, your guide circle should be 110 mm diameter for a 5mm bleed, and I'd give it a .25 stroke in any color you like, but ID by default uses red for built-in bleed guides, so red would be instinctively familar to you when doing the layout.
I said yo should laock this circle and not use it, but yo are going to have a single background object that fills the entire bleed area there's no reason you can't disregard that bit of advice and put the background into it.