It's in LASORS under section E Instrument Rating. Do you know where to find it online?
SE IR(A) to ME IR(A)*
a. 5 hours instrument time under instruction in ME
aeroplanes, which may include (b);
b. 3 hours in FNPT II or Flight Simulator if approved
by CAA;
c. Pass a ME IR(A) Skill Test with a UK CAA Staff
Flight Examiner.
* The holder of a SE IR(A) wishing to upgrade to ME
IR(A) shall either hold a multi-engine type/class rating,
or have at least completed an approved course of
training for the ME class rating.
So in effect you'll need to do two skills test and budget for the possibility of partialling both.
I'd personally not fly the Archer unless is serviceability and suitably is totally assured.