Hi,
my first suggestion would be to look on FreeBMD for Marriages twixt Fletcher and Prichard, have you tried this ?

There are a couple of possibles given the location and dates, but I'd lean towards the following;

Surname First name(s) Spouse District Vol Page
Marriages Dec 1920 (>99%)
Fletcher Harry Pritchard Monmouth 11a 93
Pritchard Lily M Fletcher Monmouth 11a 93

Then work from there, 1911 Census and 1939 Register should help.

???

Similarly for Mansel.
Not sure why you say half-brother, did their father marry wife's sister, perhaps ?

Surname First name(s) Mother/Spouse District Vol Page
Births Dec 1939 (>99%)
Fletcher Mansel S G Pritchard Ross 6a 1365

https://www.freebmd.org.uk/cgi/search.pl