Hi Mike,
Its obviously impossible to try to diagnose without seeing you but it could be that there's underlying scar tissue somewhere either where the injury was or from the graft in the hamstring. Your calves are linked by fascia to your hamstrings....and beyond so issues in one area can cause problems elsewhere. Have a look around our blog - this one might be a good intro to fascia
http://globaltherapies.wordpress.com...anating-stuff/
Where are you based? If not close to us (Glossop/Derbyshire and striking distance of Manchester) then we could help find you a soft tissue therapist near to where you live.
Lynne