I don't understand how they can refuse the hmr.
In their letter to you, they say that 'one piece of land cannot be cleared for HMR', so from this you can say that the other must be suitable for this round, ok.
You only need 'one suitable piece of land' for a particular round to be issued the certificate.
I would contact BASC and get them to help you sort it.
I had no problem with getting mine and the land is only 70 acres, 22,17hmr and 223, the firearms officer only asked me to get out with someone to get more experience using the larger calibre.
Good luck in getting sorted mate.
Happy s