[v12-engine] Misfire - compression figs

My V12 E suddenly developed a serious misfire, rough idle, poor pulling
power, spit back thro carb on stabbing a wide throttle.

took out the plugs (NGK BP5E about 5k miles old) and 3A had heavy grey
deposits all over the working points. rest of plugs looked pretty good but
perhaps a bit too white and clean. would one dodgy plug cause such
pronounced effects?

Anyway did a compression test, cold engine, throttle wide, all plugs out

B bank varied from 12 to 12.5 sound good but
A bank was
1 11
2 11.5
3 10
4 12
5 11.5
6 11
Don’t like the look of 3A which is also the dodgy plug.
What should i do - put a new set of plugs in (same make or different?) and
get on with life or do a have a future problem which would benefit from
further investigation now before it gets spendy and the tow truck comes out?

Regards

Steve.
xjs-c 85
e type 71.