Guys , Arlix and Beagle have it right . We have done several like you this summer ....If I read it correctly ; you have Easa ATPL , CPL , PPL , Living in the UK .
To gain an SEP rating , a recommendation would be a few hours training in an SEP a/c ...possibly 1 hr hour G/H .... stalls , steep turns , PFLs , settling back into a light a.c .
1 hr ccts , full flap , F0 , Glide , Bad wx , EFATO , G/A s .
1 hr setting cse on short navex , unplanned diversion , mock test of G/H + few ccts .
If all's ok , an SEP Class rating test [ about 1 hr + examiner fee ] , cse comp cert [ SRG 1107 or SRG 1119D ] + application for SEP [ SRG 1119A ] rating to the CAA plus .... money . = SEP rating on your licence .
If not done on a 172 , then club hiring 172s will probably ask for club checkout on type .
If you are down in Wessex , may I commend Abbasair at EGHA Compton Abbas , ...... yes i do instruct there part time , and we do have few understanding retired heavy drivers instructing [ from various uniforms ] ; as well as some youngsters . And several serving heavy drivers aviating on days off [ again various uniforms ]. Any of which could give you further info if needed .
rgds condor ,
P.S. Boeing keys handed back in 2009