Ground Rent Nightmare

[RANNB]

Hi, we purchased a leasehold property last year and have today received an LBA for ground rent we didn't even know was owing. It is addressed to the Leasee so not even personalised. The Freeholder refuses to speak to us and has almost tripled the bill with fees. We have received no demands or invoices, just this LBA and the freeholder claims this is because the Notice of Transfer was not served correctly by the solicitors we used to purchase the property. They say that this in itself is a breach of the lease and want a further £500 to remedy. 14 days given to remedy and the letter took 5 of those to arrive. Should we just pay up or is there a better way to deal? The purchase solicitors advise they did send the Notice of Transfer - to to the management company not the Freeholder direct though.