@Richard MacManus

Some fediverse software already has tools to link and aggregate your different fediverse identities together -- as well as provide nomadic identity and single sign-on between compatible instances, but that isn't the software you've chosen to use.

There's a specification/proposal called FEP-ef61 which brings some of these abilities to the wider fediverse over ActivityPub (we're currently using a couple of other open protocols to make these features available in the fediverse right now for those who simply can't live without it).  

Anyway, a number of folks are working on this today. Across a number of different software platforms.