A bit late for doing this, but I saw red onion sets for sale at Wilkinsons and so thought I would give it a go. I thought at this late stage there might be some money off, but alas not. Onions keep well and take a long time, so I have no worries about putting so many in at the same time. We have four square feet containing 16 per foot. That should keep us going.
At the same time, we bought a small bucket to collect water and to have by the side of the beds, as per Mel suggests. I filled this with water from the water butt and within a couple of hours in the sun was was already a lot warmer than the harsness of the tap water.