Jacksprat,

>Do you think the avenue or alignment was built around it, or the otherway round?

I suppose it depends on whether the avenue was aligned on anything. There seem to be so many erratics in the area. Kemp Howe stone circle on the outskirts of Shap village seems to have been built with similar shaped stones, and is worth a visit, although, sadly, half of it lies buried under the London to Glasgow railway.
Enjoy your visit, and keep an eye out for the many big stones beside the M6.

Regards,
TE.