pulled this from the HP Sporting website

i've been told they are about

JHM Butts & Co Ltd

Butts of Bawtry will sort you out for sure...

if butts can get the standard ones, i'm sure they can get the top ones. there the sk n's i take it

the are listed as 'practice' on thier website...

I'm pretty sure that Butts don't/won't/can't supply this variant of H&P, but the legendary John Shelley (a Czech-domiciled Brit and Skoda nut) got some of the Group N ones for me direct from the factory last year.

See his website for contact details

skofast.com

You could also try contacting the factory using the email link on their website. They do speak english, though they prefer german, and will despatch goods to the UK, taking payment by bank transfer.