Millsman, Hi!
Slipper tanks were streamlined tanks (in the Spitfire Pilot Notes they are called "auxillary blister drop tank"), what we call today "Conformal belly tank".
They were quite different from the external tanks suspended under wings or fuselage. They looked like slippers put on the underbelly of the Spitfires.
Slipper tanks existed in 30, 45, 90 or 170 gal capacity as can be seen in the following photographs:

This photograph shows a slipper tank in situ under a Spitfire in flight:
The VB series were the first Spitfires able to carry the range of specially designed "slipper" drop tanks which were fitted underneath the wing centre-section. Small hooks were fitted, just forward of the inboard flaps: when the tank was released these hooks caught the trailing edge of the tank, swinging it clear of the fuselage.
Spits with the ability to carry the slipper tank began to be issued in about March 1942
The 90 gal tank was more or less restricted to the Med, i.e. Trop Spits, at least initially, while the 30 gal was used by the Home squadrons.
the 30, 45, and 90-gallon versions were used on fighter missions with the 170 gallon tank reserved for ferry missions (and for recco missions on the PRXI).
The 90 gallon one was disliked by the pilots because it made the Spit unstable and difficult to take off.
The drag penalty imposed by slipper tank carriage was relatively high compared to the later "torpedo" ("cigar") style 45 gal drop tanks that were mounted on struts clear of the fuselage. Compared with the slippers, the "torpedo" drop tanks were little used.