What a doubling clause actually does
The lease fixes a ground rent and then multiplies it on a stated anniversary — commonly every 25 years, sometimes every 10 or 15. The arithmetic is entirely predictable, which is both the appeal of the clause to a landlord and the problem with it. A rent of £250 doubling every 25 years is £500 after 25 years, £1,000 after 50 and £2,000 after 75; on a 125-year lease that is four doublings before the term runs out.
Nothing about this is hidden or unusual in the sense of being rare. The clause is in the lease, it will be reported to you, and it was in the lease when the seller bought. What has changed over the last decade is the reaction to it — from lenders first, and from buyers once lenders started asking.
The Lexsure master wording makes the point that matters most and is easiest to miss: the clause has to be considered against the requirements of the lender on <i>this</i> purchase, and a future buyer’s lender may apply different requirements. Being satisfied yourself does not dispose of the question, because the person you eventually sell to has to satisfy their own lender, not you.