Using the protective conductor of another circuit is a no-no. For
testing it must be possible to disconnect the whole circuit at the
consumer unit.

Well this is not entirely true. Whilst the earth should indeed be taken back
to the consumer unit, there is no requirement to be able to reliably
disconnect the earth at the consumer unit. This would be very difficult in
some cases, such as a bathroom supplementary bond between lighting circuits
and socket circuit for example.

New T&E back to the consumer unit is the way to go (or singles if the
original is in conduit).

Christian.