I bought my fabia 2 days ago and have broke the stereo, is it possible to swap for a vw double din unit ?

Clumsy, anything is possible, however there can be coding issues with OEM head units. Better to get an aftermarket unit.

Guys selling a double din head unit out a vw and looks as if it would fit. Its just he's 30mins away from me how do you tell with the serial numbers

It will fit just fine but it won't work because the coding will be wrong.

The guys said he's got the code will it work

I'll be amazed if it does.