Rozy,
I think Lydia & Louisa are the same person as on the 1891 & 1901 census she states she was born Handcross & the age is about the same.
The KN Bletsoe found in 1901 as working as a servant in London is a bit of a mystery as she also appears on the 1901 as living with her parents

I can only assume she was listed with her parents by mistake

Well done, Phil

The Institute in Cardiff in 1901 - 26 Romilly Crescent, Canton, Glamorgan (part of west Cardiff) RG13/4985/162/8, does indeed list Mabel (to me it does look like BLETSOE) age 16 born Caterham, Surrey. The school was run by Elizabeth W. Melville a 72 year old widow born in Scotland. The school had 18 pupils, so quite small.
I went back to 1891 & found her husband, Alexander Melville age 70 born Carlisle running the school with 28 pupils & Elizabeth listed as the matron.
Doing a little bit more research it would seem that Alexander Melville is in some way related to Alexander Melville BELL & Alexander Graham BELL the pioneer of the telephone but I couldn't find definate connections.
jane