thanks for the reply.
to be clear the steam overlay (shift + tab) and the nvidia one (alt+K) bring up 2 different overlays on my PC - but they are both enabled!!(but never heard anyone with a similar problem mention the nvidia overlay as being required to complete a steam purchase)
also tried the logging in and out of the steam overlay several times (still no buy button).
a few users with the same problem mentioned having incomplete purchases stuck on their market place accounts and when they were cleared the market place functioned again - but have not tried this yet…