Got a reply from Gamebore, it doesn't look like the sub-sonic lead alternative will exist for the foreseeable future, it does rather call into question the knowledge, competence and indeed the motivation of the so-called shooting organisations:-
Good Afternoon,
Thank you for your enquiry.
Unfortunately we don’t have a product that meets those specific requirements – the powder we use for our subsonic lead loads will not work with steel shot.
Also, we don’t have a twenty bore biodegradable wad cup to contain the steel pellets to prevent them from coming in to contact with the barrels of the gun. (Only plastic wad cups)
Over time this situation may change as we work towards the announced three-year lead ban, however we do need to prioritise the development of the non-lead loads that will be more in demand – any products considered niche in nature (such as a subsonic, twenty-bore, steel, non-plastic wad load) might potentially follow after that, providing that the powder manufacturers also work towards the development of new, steel specific powders. At the moment it’s too soon to know what will become available.
Kind regards,
Lydia Abdelaoui
Gamebore