[arch-general] gtk related packages reported as corrupt and blocking all updates

[mick]

for the last few days I have been unable to apply updates.

Pacman reports the following:-

(85/85) checking package integrity                 [######################] 100%
error: at-spi2-core: signature from "Jan Alexander Steffens (heftig) <jan.steffens at gmail.com>" is invalid
:: File /var/cache/pacman/pkg/at-spi2-core-2.18.1-1-x86_64.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n] 
error: gtk-update-icon-cache: signature from "Jan Alexander Steffens (heftig) <jan.steffens at gmail.com>" is invalid
:: File /var/cache/pacman/pkg/gtk-update-icon-cache-3.18.2-1-x86_64.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n] 
error: gtk3: signature from "Jan Alexander Steffens (heftig) <jan.steffens at gmail.com>" is invalid
:: File /var/cache/pacman/pkg/gtk3-3.18.2-1-x86_64.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n] 
error: failed to commit transaction (invalid or corrupted package)
Errors occurred, no packages were upgraded.

can anyone help me find the answer?

mik