As the appearance of the garden has a modernist style with the straight lines and concrete furniture how about a Mondrian influence and add a bold strip of blue? It is a pond and with the right shade it could look really relaxed and contemporary. My daughter's suggestion is a chalky lilac!

I like @kelly's idea of picking a lovely fresh color of paint for the door. You can then bring it down onto the stair risers. Either a solid color to match the front door, or go wild and add some decorative elements to the risers with stencils. Or think of staining the door a great color and coordinate the stair railings and risers. I have an idea book put together for just such project that might spark some ideas for you. http://www.houzz.com/ideabooks/16700983/thumbs/Kathryn-Delany-s-Decorative-Stair-Risers

All the suggestions above are perfectly good. However, when you have a house that is entirely painted, the options are so endless that it's easy to get stuck. I'd advise either finding photos of comparable homes in colors that you really love, and reproducing a similar palette on your home… or upload a picture of your house to a colorizing site, and play around with various combos. Sherwin Williams has a Color Visualizer that is decent. I don't mind a white house, but feel that the black trim is a bit harsh. On the railing especially- it feels like it is outlining itself too much. Reduce the contrast between the body and the rail and it will enhance the house rather than overwhelm it. You have a lot of choices, so just experiment and go with what resonates with you.

Thanks for all the great ideas everyone and sorry I didn't explain the polycarbonate a bit better :) This is along the lines of what we'll be using... http://www.psp.co.nz/building-brands/corrugated-sheeting/suntuf-polycarbonate The area is an absolute sun trap and we're hoping to use it as an extension to our living space, especially on rainy days! I've just sent my husband out to get a few pots of different stains for is to try - darkest brown (cedar) and a lighter brown (rustic oak) both by Cabots + the black we already have. Hopefully we can decide between one of those! I probably should have mentioned we will have black aluminium pool fencing running along the top of the second retaining wall - so the real question is do we keep everything black (pool fencing, horizontal fence, retaining walls and pergola) or do we go with a dark (or even light) brown for the pergola and retaining walls.... We just keep going around in circles at the moment. Decisions, decisions! Thanks again :)