I don't think I would pay £4.99 for a 100g box of seed when lentils from a healthfood shop or supermaket grow so easily.
Start them indoors 2-3 weeks before your last frost date, soaking them overnight before planting seems to help them germinate faster. Plant seedlings 5" apart with rows 18" apart. They like well drained soil, not too rich or you get all leaf no pods. The plants grow to 18"-24" tall & benefit from some support (pea sticks). Pods mature in August/September. Yields are low, on average you only get 1-2 lentils in each pod with 100-150 pods per plant.
IMHO they are more useful grown as a green manure than a foodcrop.