Ive managed to buy an ex-demo I/R Jagd and accourding to the shop it came from it had never been used

it came with all of the parts I/R mushroom etc and has a bulb in the barrel but it wouldnt flash so
I opened it up and the cable from the RX18 to the socket in the turret for the I/R was missing so I replaced it with one from my stash of cables only to find the flash doesnt flash

any idea's

or can i just replace the flash thats there with a new unit

and how do I know if it has the I/R emmiter fitted what does it look like

Cheers for any help in advance
Pete