A lot depends on the vintage of the machine. If it is old (roughly 1950's or older), then it'll have the old hard to find spindles that use 5/16" ball bearings. But if later, then much easier, solo size  1/4" bearings.

If you can post pictures, then can probably tell from these.

Sorry for the late reply, hectic week, I've uploaded some pics of the problem on Imgur, link: https://imgur.com/a/Wnozxqm

The smallest ring overlaps the stoker bottom bracket so it can't turn, I was just wondering if someone could give me a rough estimate of a bottom bracket length that might help, or failing that another fix to help it clears the frame, I haven't even played with chain line yet ;-;

Thanks,

Jordan~

Chain reaction do a shimano UN55 68mm wide 118mm BB which should give 2.5mm extra each end of spindle. austen

Hi again. 

I see you have a 73mm BB under stoker. Check width of B.B. shell/housing as you will prob need a 72/73mm wide B.B. and not a 68 as I mentioned above. austen

Ok great! To recap for my own sake, go with a 73mm bb that has a 118 spindle? Would I need a similarly long front bb as well, for a straight chain line, or would the current 113mm be alright? 

Much appreciated on the responses though,

Jordan~

I would leave the 113 BB at the front as front chain line should be fine. The eccentric front B.B. prob has some lateral adjustability if needed.

Austen

Ok, thanks ever so much, really appretiate all the help, ordering a new bottom bracket as we speak, so I'll let you chaps know how it goes.

again much appretiated,

Jordan~
 

That worked a treat, it's on and everything is working smoothly so far, thamks very much for all the help! 

Jordan~