To answer your question , yes there are very significant differences between the NOS and the current Mullard branded tubes.
The current "reissues" are made in Russia and evidently come from the same manufacturers as do Sovtek, they are now called New Sensor. The good news is that the new Mullards and Tung Sols don't sound like Sovteks. They are much fuller,less dry, have much less glare or treble brittleness. They are indeed very very good and its nice to know they are around.
The old Mullards posess an incomparable and indescribable warmth and richness with a perhaps a small emphasis on the midbass (the opposite of a thin sounding tube). The old tubes still preserve detail and have treble extension but are very sweet, non fatuiging in the upper registers -mellow toned if you will. If you can afford some like the fairly common, later CV-4004s, I encourage you to try them. Don't forget the Gold Lion KT 88s you will be thanking me for that recommendation, you gotta believe me on this one.