Quick update on this.
First installed a UK (QWERTY) keyboard, which made life a lot easier. Annoyingly it's not an
exact match (physically) for the German one ... there's a gap of a mm or two all round. But it's not really that noticeable, and all works fine.
Downloaded a Win 7 Pro 'upgrade' (from XP or Vista) from software4students - thanks A-Avantgarde. A bit of fiddling around required as it was in .img rather than .iso format, so had to burn to a DVD (rather than using USB stick).
A colleague at work advised that you don't actually have to have XP or Vista installed to use these 'upgrades' - you just need to insert an original disk at the appropriate point. So I ran the install within the German copy of Win 7 ... all went fine and guess what - it never asked for an XP or Vista disk! So I could have saved a bit of money there.
A few issues with the hardware - no WiFi for a start. So cabled it direct to the router to activate the copy of Win 7 and start applying 50+ Windows updates. Got that all done and Microsoft Security Essentials installed (for now), and it all runs pretty well.
Device manager showed an 'unknown' network controller with no driver which I figured was causing the lack of WiFi. Off to
HP - Deutschland to look at drivers, guessed at a Broadcom one which seems to have done the trick (need to confirm tonight that I can actually connect via WiFi). Some issues with audio too (mute/vol up/vol down on keyboard not working) so again downloaded a driver from the German HP website which seems to have fixed. Not tried the wireless mouse yet, I'm guessing that will need a bit of tinkering too.
But it's looking pretty good so far - thanks for the help & suggestions guys!