I went into my local Morrisons and found 4 packs of Braun Replacement heads for £7.95!
I took a picture (which got a few looks, will upload it later)
* Pack of 4 replacement brightening toothbrush heads
* Special polishing cup maximises the effect of toothpaste to deliver superior polishing action, while outer bristles surround tooth
* Interdental tips clean deep in hard-to-reach areas as well as below the gum line
* Indicator bristles fade halfway to signal when to replace your brush
* Suitable for all Braun Oral-B rechargeable toothbrushes except Sonic