Apply discount to certain products when another is in cart

I'm looking to apply an automatic discount to specific products when a certain product is added to the cart

For example: "Get £5 off any of our books when you purchase a t-shirt".

Could somebody suggest a module to do this?

Thanks

I dont believe there is a module for this yet. I have it on my list of "modules to make" but that list has taken a back burner to porting my current mods to 1.5.x. So in 2 weeks I will have time to make one