Here's something I didn't know...
If you defer your State Pension, and then opt for the lump-sum, it takes months before they actually pay out. No big surprise - this is a government department, after all.
But I've now discovered (a) they add no interest to the lump sum after the date you applied, and (b) it is actually official DWP policy to delay paying the lump sum...!
The Pension Service claims the "policy" delay is 6 weeks - but, of course, in practice it's far longer... after 6+ weeks, they simply send yet another letter asking *again* if you actually want the lump sum.