Well, I've finally cracked it. Nothing clever on my part and your hints pointed me in the right direction. I replaced my ActivePerl build 631 with build 633, retried the ppm install and it all worked fine!

I noticed as part of the upgrade that my $Archive::Tar::VERSION changed to 0.072 (I swear it claimed to be 0.22 before the re-install - bizarre).

Cheers.