I decided to try the third Mullard in the Rectoverb's first stage and it kept cutting out. Tried the second one and all is fine. According to the schematic only half the third tube is used in the Lionheart, so its probably suspect for any other use.
I've now got a Lionheart with Mullard first stage, Mesa first stage tube in the second position, and Juansolo's Sovtek from his Blackstar in the third (he doesnt know I've nicked it yet :) I didn't take out of his amp, may have to try this suspect Mullard in the Blackstar.) Still sounds great, and maybe has a little more gain.
The Rectoverb now has a Mullard in first stage, and sounds much nicer when clean, warmer and less sterile, dirties also sound better, but I need a little more gain, for the same output as before.
If anyone's wondering why I didnt check the tubes in the Lionheart before, it because the first stage tube shield is right against the side of the casing, and basically impossible to turn unless you have the fingers of a 5 year old.