At JLW,
That beer is huge to pitch an English ale yeast onto, especially if the starter was shy of estimated yeast cells needed. In my experience, English ale yeasts flocculate really quickly, and, when a recipe is high in speciality malts/decoction/high mash temps/high gravities, in whatever combination, English yeast flocculate the beer before primary fermentation reaches complete attenuation to the potential FG, and that is because of the complex carbohydrates created in these processes which take longer to convert. You were clear of most of these, except for high OG and low pitch rates of English yeast. It kind of stalls when it reaches those high alcohol levels and the yeast are tired and settled, barely coverting to alcohol at a really slow rate.
And, I had the exact same thing happen to me on an Imperial Stout I made a couple of years ago. It stalled at about 1.038, should have finished lower, or at least I hoped. But, I did not pitch enough, used English Yeast, and fermentation temperatures dropped at the wrong time. Needless to say, fermentation was over at that point, and re-pitching yeast on a beer with alot of alcohol and one that is partially fermented is probably not going to work, even if you use a starter and over pitch. I tried several things, I roused the yeast cake first and raised the temperature....nothing. Put the beer in with a fresh yeast slurry from a recently completed primary....nothing. I repitched on that too....nothing.
After lots of research and frustration, I have found that if a high gravity beer stalls, it's really difficult to get it restarted and drop that gravity further. It is because in the fermentation process, its kind of like the snowball effect; the yeast are rapidly reproducing and developing enzymes to chew through carbs and make alcohol. They become acclimated to the constant increase in alcohol in solution and have the momentum of growth to continue conversion to alcohol, and once that stops or stalls it's over. The yeast have done all they are going to for that go 'round. The only theoretical way to get it going again would be to pitch a huge starter. One in fact that would be a second batch of beer. At high krausen, put around 10% or more of the fresh fermenting wort into the stalled beer. It will take off and finish if everything else is right, but that is difficult with limited facilities and considering how much it would affect the finished flavor, meaning that it would have to be a complete brew day just to make that "re-starter" beer of the same style.
Sorry for the spiel.....
Let it sit for a little longer, then take a good sized sample,sit down, and drink it. If its good, and FGs are constant....bottle or keg it.