Each week;
I take a full weekly rest from Sat 08:00 to Mon 17:00.
I then take reduced weekly rest from Wed 08:00 to Thursday 17:00.
I understand, I think 
, about reduced daily rests and compensation for weekly rests from posts previously on here.
My question is; Does the right to extend my driving hours to 10 hrs in one 24 hr period twice in a week also refresh with the new working week on Thursday evening or must it remain as two periods within the fixed week Mon 0:00 to Sun 24:00 ?
No, your two extended driving days only reset at 00:00 hours Monday. Your three available reduced daily rests reset, in your case at 17:00 Monday and 17:00 Thursday.
Thanks Coffee, 