You've sure got the 'sandy loam' requirement down. Nice soaker hose layout, there!
I'd suspect the diff between Kent's climate zone and yours (and mine) is that the nights don't cool off nearly as much in TX and FL as they do in NB. The plants just don't recover as much over night. It's the same reason my tomatoes give up sometime in June here - even though the days aren't that hot (93 was our hottest temp the last two summers) they need cool nights to revive.
Maybe try watering in the evenings to cool the soil and let the plants drink up over night? Just enough to get the leaves perked back up to survive the next day's baking. The wilting isn't a concern as long as the leaves recover over night and look good in the mornings. You need lots of healthy leaves to make sugar in the melons.
Come to think of it, the commercial melon growers here do harvest in late spring, so they probably have trouble keeping the plants going through summer too.
