You need organic matter. Lots of it. Spend a dollar or two on a 40# bag of compost, and dig that in. Fixing clayey soil is not expensive, but it does require some digging.

Agree that it sure can't hurt anything but doubt it will help much. Be sure to mix it in well to gain any benefit. If you will provide the dimensions of the garden we can likely advise on how much compost would be needed annually (it isn't a 1 time only addition).

Dave

Thanks for all the replies :)
Determining the amount of compost needed would be very helpful, thanks.

The dimensions are: 185cm x 212cm
or 1.85m x 2.12m
or 72.8" x 83.5"
or 6.1ft x 7.0ft

Really need a depth estimate to compute volume but assuming 12" depth of turned soil you have approx. 42 cubic feet (or less depending on depth). Depending on how the compost sales are measured there (here it is by cubic feet) I'd look for 10-12 cubic feet of good compost to work in several weeks prior to planting this year and then add an additional 2-4 cubic feet annually. That amount will vary depending on your climate/location as that will determine how rapidly it will decompose.

As your soil bioherd develops you may need to space out the additions 2-3x a year.

Hope this helps.

Dave

Your proposed bed is actually only about 4 sq m and could easily be thoroughly double dug and amended in an hour or so. I'd start with about 10 cm depth of compost over the whole bed. I think your first year should be a low cost experiment. You may be surprised at what grows. So far you don't actually know that your soil is poor or badly drained, you are just worried it might be. This calculator might help you Amendment calculator.

Yep. Clayey soil needs about 20-30% good quality compost double-dug in. Best to get it professionally tested at some point to identify nutritional deficiencies that might not be conspicuous. If you have nearby gardeners who have done that with the same kind of soil, the analysis that they got will probably apply to you.

We have clay here in the US. My friend made a clay tennis court in his yard. I've learned to minimize my work and increase yield. You can see trees weeds and grass growing in Clay. You can just ammendment the area where you will put a plant. I make a hole with a trowels add a handful of the potting soil mixed with fertilizer and a little lime and mix with soil. You can do the same with a row for seeds. Mulch around and let the worms carry it under. You probably have enough for this method. I have been amazed at the yield. Over time it will get better. Save your back and enjoy gardening.