Rubber Washer for a Shower Tap

[TW]

The rubber washer that sits inside my mixer/Shower tap for the bath has died a death so I can no longer switch between water coming from the shower head or the tap. It's just a basic non-pumped item with a plunger you pull to divert the water from one to the other. Problem is that it's an odd shape, looking like a diamond shape in cross section I've tried modifying a tap washer, but it seems it won't work unless the shape and size is exactly right. There are no brand markings on the tap and it has been there since I moved in. I've looked in all the normal hardware chain stores but they don't seem to have anything like it. Changing the tap unit is going to be major ball-ache due to the way the bath has been put in. Any suggestions of anywhere to get such a washer, either on-line or in the Kent area very much appreciated.

Ta