Ok so i think i'm getting somewhere with this so thought i'd add where I am on the off chance anyone else has the same question in the future.
So while the collet chucks i have, have nothing written on them to tell me what collets they are designed for I have managed to establish the following
1 the chuck on the mill has QC30 written on it
2 The clamp I use to hold the collet chuck still while I clamp the collet into the chuck has the following written on it
Based on this it would look like I can buy a 30INT-ER25 collet chuck and i'll be able to use my ER25 collet on my Interact.
Does that sound about right?