Clearly inspired by the T21b Sedbergh, but with some differences. Here's the whole aircraft. Note the dive-brakes rather than hinged spoilers and the different fin and rudder.
Grunau Baby was single seat however the T21 series was developed by Fred Slingsby based on it.
I remember a joke along the lines of Slingsby mis-reading the plans for the Baby, thinking the dimensions were inches rather than centimetres!