Speaking of that, is Miller a maiden or surname?
My advice is first go to the hospital
, and then to county to make certain you have every possible record and document. Did they have a business license, driver license, buy a house, window washer insurance?
Interesting to note ... Middlesex was a separate town and county until London annexed it in 1965 ... yer lass was nary a wee sprog. These kinds of records might be elsewhere, other than whatever main London annex you might have accessed.
Definitely try the office records of physicians who treated her heart condition, they should be separate from hospital records. Often they request fairly detailed family histories ... helps them diagnose and treat. And it's a longshot, but this is a women's networking group
in Middlesex. Chicks dig helping other chicks they've never met here, probably there too.