It's 32 hours plus 2 x tests of about 1 1/2 hours each.
From LASORS:
Upgrade to JAR-FCL PPL(A):
The holder of a NPPL with SSEA Class Rating wishing
to obtain a JAR-FCL PPL(A) shall :-
a) Complete at least 45 hours flight time, of which at
least 35 hours must have been as a pilot of SEP
aeroplanes. A maximum of 5 hours may be
completed in an approved FNPT or flight simulator.
This flight time must include :-
1) 20 hours dual instruction of SEP aeroplanes with
a JAR qualified instructor;
2) 10 hours supervised solo flight time on SEP
aeroplanes which must include 5 hours solo
cross country flight time including one cross
country flight of at least 270 km (150 nm),
during which full stop landings at two different
aerodromes different from the aerodrome of
departure shall be made as per JAR-FCL
1.125.
b) Pass the JAR-FCL PPL(A) Theoretical Knowledge
Examinations (unless previously passed for the
NPPL (SSEA).
c) Hold a valid JAR-FCL Class 1 or 2 medical
certificate.
d) Pass the JAR-FCL PPL(A) Skill Test as detailed in
LASORS Section C1.4.
Note that this doesn't mean that you have to fly an additional 45 hours, it just means that the hours you fly post-NPPL when added to your NPPL time must total not less than 45 hours including the individual requirements as above. Any 'solo' flying must be with no-one else in the a/c irrespective of your NPPL privileges - you may not fly your 150nm cross-country with a friend, for example.