Just use the "active" input for now and turn up the "gain" control to compensate. It won't hurt anything to plug a passive bass into the active input.
...it looks like a small square plastic thing soldered onto a small board.
This describes most electronic components.
I presume you mean the input jack. Does it look something like this?
I don't know where in UK to buy these, but the common name for this part is a "PC mount 1/4 inch input jack". I'd take the old one out to make sure the replacement's pins line up the same as the old one.