Tecnik TDK 795/2 dishwasher...help!?

We are trying to fix the tablet dispenser where the sliding door won't open at end of cycle and tablet still in there. Can see that spring is broken. Can we just replace the spring or do we need whole new dispenser unit? If so, how do we get it out and where do we order parts from?
Also found white plastic cog under the dishwasher, about 2 x 1 cm. Any ideas where this could have come from?

Tracey and Phill Burton
April 2012