how do i measure what size speedo cable i need? it will be a speedy cables one as it will need to fit the vdo speedo in the buggy and a standard vw one wont fit at the speedo end

I would measure up or send an original one then ask them to do one with the VDO top end, I guess the inner cable square may be the same but the nut or fixing will probably differ.
you may be able to get an adaptor cable to head.