Help training shy Koi

About 6 weeks ago I restocked the pond with some young Koi (~2" mixed
colour mostly but two 4" ghosts as well).

I appreciate that fish usally take a while to become acclimatised to a
new home but they have been constantly hiding, mostly grouped together
underneatha patch where the liner overhangs. They ignore all food
(there is plenty of crap on the bottom for them to eat a this time of
year though). By this point, late spring, I'd have expected them to
start coming out of their shells a bit and maybe taking the odd food
stick.

Does anyone have any advice as to how I could help calm them down a
bit (they are in no danger from predators, its a steep walled central
London pond so no Herons/Kingfishers etc.). Water lilies are just
starting to open and I imagine these will help.

I'm not worried aobut their health, they seem well and active when
spooked.

Cheers,
Ed