Packs of 4 Colgate ProClinical triple clean refill brush heads are £18.00 and Buy One Get One Free In Waitrose.
** Post amended after feedback:
These packs are also in their Pick Your Own (PYO) offers and if you pick them you get a discount of £3.60. So 8 brush heads for £14.40
I originally thought there would be a discount of £7.20, but this doesn't seem to be correct. Thanks to those who have pointed this out.