This is a very straightforward task, requiring only one tool - a big screwdriver.  Flavia handbrakes have attracted some adverse comment in the past.  It is important that all pivots and brake cable pulleys should be free to move: worn pads should be replaced - there is a URL for a Jaguar website supplying these in the Alternative Parts page, via the link above.  Cables should be neither frayed nor slack.  Once all this is correct, the poor reputation of this type of handbrake seems quite unjustified.

Stand the car on level ground.  In 1st gear, drive the car slowly forward.  Obtain the services of a volunteer, who will be looking through the holes in one of the rear wheels (on the rearward side) in order to sight the big slotted screw heads shown in the photos above.  It is unlikely that the wheels will be aligned such that both O/S and N/S adjusting screws will be visible at the same time.  These are positioned at approx. 7 o'clock (O/S) and 5 o'clock (N/S) - the pictures above mislead slightly.

Once one adjuster is located, stop the engine, leave the car in 1st gear, chock the front wheels, but do not apply the handbrake - which presumably isn't working anyway.  With your big screwdriver, turn the adjusting screw clockwise until, with nothing more than firm force, it won't turn any more.  Then rotate the adjusting screw anti-clockwise half-a-turn to loosen it slightly back from where it had stopped.  Repeat for the other side. 

That's it.  Job done.  You may be quite surprised how many turns clockwise can be managed before each screw stops, but it has a fine thread, and all that rotation will only represent a few thou.  If all the other variables mentioned are OK, the handbrake will now work just fine.  You will be amazed...