I am giving the nematodes a go this year on the spuds too and have just ordered the second box having applied the first early June. But I am not sure how well they really work (in spite of all the rave reviews on Unwins's site) as some of the Lady Crystl spuds I harvested were full of holes. Still, you've got to complete the experiment or the data is useless. So yes, the instructions on the box say 6 weeks so you need to fork out again for the product, unfortunately.
It can't hurt to apply plant food to the crop, depending on when you planted you should be seeing lots of foliage but as long as it looks healthy, that is the main thing.
Disclaimer: I know nothing.
Qualifier: Actually, neither does anyone else.