You should be able to take a single skills test on the type you are currently rated on. UK CAA LASORS Section G refers:
ATPL(A) holders who have a minimum of 1500 hours flying
experience as PIC or Co-pilot on multi-pilot aeroplanes
(or single-pilot aeroplanes operated by 2-pilots according
to operational requirements) and hold a valid multi-pilot
type rating for the aeroplane to be used for the ATPL(A)
skill test and have at least 500 hours experience as pilot
on that type, will be exempted from the requirements to
complete an approved TRTO course or undergo approved
training prior to undertaking the theoretical knowledge
examinations and the skill test.
The ATPL(A) Skill Test in this case will have a 12 month
validity for the purpose of licence issue.
Pilots who meet the 1500 hours flying experience (as
detailed above), with less than 500 hours experience as
pilot on the type to be used for the ATPL(A) Skill Test,
or do not hold a multi-pilot type rating will be exempted
from the requirements to undergo approved ground
and flying training prior to undertaking the theoretical
knowledge examinations. In both cases, they will be
required to complete an approved JAA type rating course
and if applicable, the course may be reduced to take into
account any previous experience on the same type upon
recommendation by the Head of Training of the TRTO to
PLD. An ATPL(A) Skill Test with, or observed by a CAA
Flight Operations Training Inspector will be required at
the end of the TRTO course. The ATPL(A) Skill Test in
this case will have a 6 month validity for the purpose of
licence issue.
You'll need to find a JAA TRTO and liase with your local CAA to have an examiner observe the test.