"get_iplayer is not writable" on Windows 7 x64?

[Christopher Woods (CustomMade)]

Apologies if this has been discussed before, had a quick skim of the list
archive prior to subscribing and couldn't see this mentioned.

I installed get_iplayer on my W7 install and was greeted with this lovely
warning, then error, when calling it for the first time via my custom
string:

WARNING: Running the updater again to obtain plugins.
ERROR: get_iplayer is not writable - aborting update (maybe a package
manager was used to install get_iplayer?)

The string I use (and which works fine for radio in XP):

get_iplayer --raw --output F:\iplayer-j\get_iplayer\raw\ --modes
flashaachigh,flashaac,flashaacstd,flashaudio,flashaaclow --rtmptvopts
"--swfVfy http://www.bbc.co.uk/emp/10player.swf" --rtmpradioopts "--swfVfy
http://www.bbc.co.uk/emp/10player.swf" --force --pid 

This is entered at a DOS prompt beginning in the get_iplayer dir. Aware I
most likely don't need --rtmptvopts for radio but I started with a TV string
(and it's always worked)... It also doesn't work irrespective of whether I
set XPSP3 Compatibility Mode or forcibly install to Program Files (as
opposed to Program Files (x86) the installer defaults to). All the batch
files have relative paths in as far as I can see... Just calling get_iplayer
to invoke the PVR also fails similarly.

Has this problem been encountered before and if so, what's the workaround?

TIA
Chris