Be aware that Marlow Excel Control Line and Robline Dinghy Control Line are constructed differently. The splicing methods for the two are therefore different as well.

Marlow Excel Control Line has the strength in the cover whereas Robline Dinghy Control Line has the strength in the core. For the Marlow, you splice the covers together. For the Robline, you splice the cores. Splicing the wrong part together (on either) will significantly weaken the line.

To make a continuous loop in core-dependent line, I generally do a stripped-cover splice in each end and then end-to-end splice the cores together. That yields a continuous diameter splice, but the core is partly exposed.

If I want a fully covered core and can tolerate a 50% increase in line diameter, I'll end-to-end splice the cores, milk that splice back into one end, and bury the covers at a slight offset.

I hope that helps,
Eric