Very dissapointed to find that the blaupunkt speakers that i bought the car with had dry rotted. They sounded surprisingly good considering the size and the standard panasonic head unit. Any suggestions on 6x4s to take their place.

I got a set of the jbl ones off eBay for £50

Or for a bit less money the Alpine SXE-4625S are quite good  

I got mine out of a daewoo matiz, they where free and I am very happy with how they sound. 

 

Also, from the pic the speaker looks like it's the same as the front ones but with a different base attached. Those are prone to rotting away.