The replacement valve arrived today and it working well and seems to have resolved the noise issue. As a bonus I managed to obtain a NOS vintage Jan Tung Sol Black Glass valve which sounds wonderful.
I exchanged a few emails with Nathan of Sushibox and he did offer to refund me the cost of a replacement valve but I told him not to bother as I would have started tube rolling sooner or later anyway. He did say he is planning to move from Sovtek to JJ valves in future.
I had initially bought the Grand Slampegg with the intention of using it on a few songs I thought would benefit from a vintage tone but, after playing it for a while, I think it's going to be an always on pedal.