Yes Dan if you have time and materials to hand, it is worth refilling for those heat-loving plants.
However April is looking to be much warmer than last year and I decided yesterday simply to put two barrowloads of fresh manure on my hotbed in the greenhouse, with its core temperature still an impressive 40C, from the initial fill on January 29th.
Yes it is good to sow melon and basil now, other cucurbits a little later, mid April is good, even late April for courgettes in case of frosts up to mid May, and cucumber for outdoors is best sown mid May.