Update: GRO have refunded part of my order (for the birth and marriage cert of Eliza). They seemingly have found the marriage – I’ve not got it yet, but they couldn't find her birth. (I'd ordered that birth certificate based on me hoping that the Took name was really Tuck).
GRO response:
‘There is no trace of the above mentioned person at the reference you quoted.
Eliza J Tuck        1883   Dec   Lanchester   10a   292 ‘
Wondering now, should I have named her as Took which is the name on the record I found on Ancestry, even though I’m hoping she’s Tuck?  If I do, and they find her as Took, can I assume they will see an alternative spelling on their records? If it remains as Took, dare I assume she is my great grandmother?
 
Think I’d best wait to see what the marriage cert throws up, if anything……. 
