Ground rent purchase - cheapest way?

Im paying annual ground rent to a company called Birdfield Securities on my PPR. They always enclose a ground rent purchase plan representing a 24 times annual rent plus a small handling fee ( using their solicitor I assume) to buy it back.
I intend to purchase this outright soon and my question is:

Is this the cheapest way generally of doing so, or should I go through the Land Registry ( seen their site), or use my own solicitor?

I presume that you've already see the and Land Registry links on this?

Thanks Clubman,
Hadnt seen the OASIS site.
My main query is whats the most cost effective way of doing this?

I would imagine a DIY job with the Land Registry (just like vacating the deeds on a property whose mortgage has been cleared is cheaper by going direct than through a solicitor) but I don't know what the caveats might be (e.g. what possible legal issues could be overlooked). I'd certainly investigate the DIY route first before involving a solicitor myself. Perhaps the Land Registry links outline the sorts of things that need to be taken care of and if they need legal input?

Don't know if you have purchased your ground rent from Birdfield but we did, through our solicitor in 1999. Yesterday (2009) we received letter from Birdfield threatening legal proceedings if we didn't pay GR due - which, as we and our solicitor have pointed out to them over the years, is not due as all amounts were cleared in 1999. we have given them details of the Vesting Cert etc but still they continue to hassle us. At this stage we don't know what morw we can do. They don't reply to letters and never answer the phone. My advice - make sure you use a solicitor with this company.

If the land—
(a) is not used for the purposes of business,
(b) is held by the person acquiring the fee simple under a yearly tenancy, under a statutory tenancy implied by holding over property on the expiration of a lease which reserved a yearly rent or under a lease which will not expire within fifteen years after the date of the service of the notice under section 4 of the Act of 1967 or of the application under Part III of this Act, and
(c) does not exceed one acre in area,
then, subject to the subsequent provisions of this section, the purchase price shall not exceed the amount which, if invested on the date of the award in the security of the Government which was issued last before that date for subscription in the State and is redeemable not less than fifteen years after the date of issue, would produce annually in gross interest an amount equal to the amount of the rent payable under the lease or yearly tenancy, as the case may be, during the year immediately preceding the date of the service of the said notice or application.