The underlying licence has to have the underlying ratings valid to be used with a 61.75. The FAA have then deemed that in the case of a medical as long as you have either a medical to match the underlying licence or an FAA medical then you are fine.
Don't confuse the medical with the licence validty.
about 20 mins airborne for the LST if its only been 5 years
I can just imagine the response from the CAA if I was to conduct an LPC/LST in 20 minutes and send in the SRG1119 with that time on it........
there is no reason why not if you are reasonably competent and prepared
Take a look at the SRG1157 and tell me how I can conduct that content in 20 mins.....