Does anyone know if we can "cash in" our UK Tax Voucher which...

Does anyone know if we can "cash in" our UK Tax Voucher which came with dividend payment ..... or is it just dead money ??
Appreciate any advice thanks,
Snoball.
ps. have applied for comsec acc to be prepared to perhaps take a little profit off the table, however, appln takes forever .... 5 weeks now & still no paperwork from comsec however rec'd acc # from Pershing USA who apparently carry out the comsec trades ??