First year at growing vegetables. Don't know anything about previous
history of the soil... it's clay/chalk.

Took it over last November and dug, dug, dug clearing as much weed as
possible and left it rough till Spring this year . Raked it lots to
remove stones and produce tilth. Now I have 5 beds to rotate in the
order: Potatoes, then Onions/Leeks, then Legumes, then Brassicas, then
Roots, then Potatoes again to restart cycle. Most crops are now in.

But not knowing soil history , how to feed plants?

I am thinking of applying ChemPak No 2 (high nitrogen) to everything
until July and then switch to ChemPak No 8 (low nitrogen , high
potassium/phosphorus) for rest of season except for Brassicas.

Legumes would get ChemPak No 8 throughout growing season as they dont
need a lot of nitrogen.

Does this make sense?

Yours...
Hairy Legs

(South East , UK)