Just had this exact same problem with my 1.9
Injector 4 had developed a leak, which initially only showed up as a damp patch on the rocker cover around the injector.
after about 3-4 weeks it had turned into a large damp patch with lots and lots of crap built up around the injector.
On close inspection it was leaking from the seal where the top of the injector screws onto the main body.
It was a massive PITA to change as the leaking fuel had run down the side of the injector and dried out and gummed up, effectivly gluing the injector in place, my mechanic (brother in law) used a proper injector remover but it required such force that it dented the rocker cover!!!
Get them checked out sooner rather than later, I bought a replacement off ebay for £70 and after some initial trouble with coding the ecu it's now running like a dream again.
I guess if the garage have already done some work on the injectors and replaced 1 they might not have retightened them properly causing blow by, If this is the case then a fire is probably quite likely, if it's the seals that are leaking then I had not trouble other than noise (it sounded like a tractor) and excessive fuel use.