heather and grass

back in april this year, i very successfully transplanted around 150 sq.ft of heather to the bank in my garden where it's now flowering with a bed of purple. should look even better next year!
however, there is a considerable amount of grass growing amongst it which, at the moment, i have to remove by hand. getting the roots out isn't too much of a problem but the whole exercise does still take some time.
it there a weedkiller that i can use to eradicate the grass and the other few weeds but that leaves the heather alone? i don't want THAT to die after all my hard efforts!
if anyone can help or has ideas about this, i would love to hear from you please. it would be much appreciated.
thank you in advance.
michael.