Nice find,
davem.
It seems to me that, by this comment, Tolkien is saying that he could only fully explain how LotR developed in his own autobiography, given that it took up such a large part of his life and that his experiences during that time were integral to its development (I suspect that its the latter point that interests you,
davem 
).
Only £1,000 to £2,000? Bargain! Although I had understood that most of the letters that have been complied and published were donated. Still, it would be interesting to see it.
Hmm, I must start watching
Antiques Roadshow again.