So where do they mention it is jettisonable?
"An auxilliary "blister"
drop tank of 30, 45 or 90 gallon capacity can be fitted below the fuselage"
"
Drop tanks should only be
jettisoned if this is necessary operationally"
There's quite a bit more in the manual, eg:
"When carried, the 90 (or 170) gallon drop tank must be jettisoned before any dive bombing is commenced"
"Except in emergency, the fuselage bomb or drop tank must be jettisoned before landing with wing bombs"
"Drop tanks should not be jettisoned unless necessary operationally. While jettisoning, the aircraft should be flown straight and level at a speed not greater than 300 mph IAS"
What troubles me is how a slipper tank behaves when it is released in flight. If it does not detach properly it could slam in the bottom of the a/c because of its shape.
Spitfire The History notes that the 170 gallon torpedo tank with tailplane was prone to damage the fuselage in trials, but not the slipper tanks. The only other problem about dropping tanks I can see mentioned is a test of a tropicalised Spitfire V, there was no indication to the pilot of when the tank detached (doesn't say what type of tank)