When to plant out my Cistus

hi gang, 

I took a cutting last year from my cistus bush (i'm afraid i don't know the variety but the flowers are deep pink and the foliage is thick and very much like sage leaves)

The original plant is now about 7 years old and has become a bit of a beast with fewer and fewer flowers each year. I think it needs to go onto the compost heap. 

The cutting i took is now about a foot tall and wide, growing outside in a pot. Should i dig up the old guy now (mid sept) and put the new plant in his place, or is there a more appropriate time of year to do this?

Thanks